Notch Signaling: Receptor Processing, CSL Transcriptional Complex, and Pathway Regulation in Research

# Notch Signaling: Receptor Processing, CSL Transcriptional Complex, and Pathway Regulation in Research

For Research Use Only (RUO). Not for use in humans or animals.

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Notch signaling is a conserved juxtacrine cell-to-cell communication pathway that governs binary cell fate decisions, lateral inhibition, boundary formation, and stem cell maintenance across diverse tissues. Unlike most signaling pathways that rely on diffusible ligands activating cell-surface enzyme cascades, Notch depends on direct cell-cell contact: a transmembrane DSL ligand on one cell engages a Notch receptor on an adjacent cell, triggering sequential proteolytic cleavages that release the Notch intracellular domain (NICD). NICD translocates to the nucleus, converts the CSL repressor complex to a transcriptional activator, and drives expression of target genes including HES1, HEY1, MYC, and NRARP. The pathway is subject to extensive post-translational regulation through endocytic recycling, ubiquitin ligases, and glycosylation. Dysregulation is causally linked to T-cell acute lymphoblastic leukemia (T-ALL), where activating NOTCH1 mutations occur in >50% of cases, and to multiple solid tumors. This article provides a mechanistically detailed account of the Notch signaling cascade, regulatory mechanisms, and the research tools used to dissect this pathway.

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Notch Receptors: Domain Architecture and Maturation

Four Mammalian Notch Paralogs

Mammals encode four Notch receptors (Notch1-4), each a single-pass type I transmembrane protein of ~2,500 amino acids. All Notch receptors share a conserved modular architecture:

Extracellular domain (ECD):

• •29-36 Epidermal Growth Factor-like (EGF-like) repeats — the primary ligand-binding interface; EGF repeats 11-12 mediate DSL ligand interaction; O-fucose and O-glucose modifications on specific EGF repeats regulate ligand binding affinity
• •Lin-12/Notch Repeat (LNR) domain — three cysteine-rich modules that maintain the receptor in an auto-inhibited conformation by masking the S2 proteolytic site
• •Negative Regulatory Region (NRR) — contains LNR repeats + heterodimerization domain (HD); the primary autoinhibitory switch

Transmembrane domain and intracellular domain (ICD):

• •Single transmembrane helix containing the S3/S4 gamma-secretase cleavage sites
• •RAM domain (RBP-Jkappa Association Module) — unstructured region; high-affinity CSL-binding interface (Kd ~15 nM for Notch1 RAM)
• •Seven ankyrin (ANK) repeats — Armadillo-like repeats that fold into a TPR solenoid; interface for MAML (Mastermind-Like) co-activator binding and CSL interaction
• •TAD (Transactivation Domain) — C-terminal region of Notch1/2/3; variable among paralogs
• •PEST domain — C-terminal proline/glutamate/serine/threonine-rich sequence; phosphorylation target for Fbxw7/SEL-10 ubiquitin ligase-mediated NICD degradation

S1 Cleavage: Furin-Mediated Maturation

In the trans-Golgi network, Notch precursors undergo S1 cleavage by furin-like proprotein convertases at the RXXR furin recognition site within the HD domain. This generates an extracellular fragment (NECD, comprising EGF repeats + LNR + HD-N) and a transmembrane-anchored fragment (NTMIC, comprising HD-C + TM + ICD). These two fragments remain associated through non-covalent interactions (disulfide bonds + hydrophobic contacts within the HD domain) to form the mature heterodimeric receptor that is trafficked to the plasma membrane.

The S1 cleavage is required for the receptor to adopt the auto-inhibited NRR conformation. S1-uncleaved Notch undergoes ligand-independent activation (endocytic trafficking activates the receptor even without ligand), illustrating that S1 maturation is not only biosynthetic but a functional lock.

EGF Repeat Glycosylation: O-Fucose and Fringe

Multiple EGF repeats are O-fucosylated at the consensus (C2-X4-5-S/T-C3, where S/T is the fucosylation site) by POFUT1 (Protein O-Fucosyltransferase 1) in the ER. O-fucose on EGF12 directly contacts the DSL ligand -- structural studies show the fucose mediating hydrogen bonds with the DSL domain of DLL4. Mutations eliminating POFUT1 activity severely reduce Notch activation.

Fringe glycosyltransferases (Lunatic Fringe/LFNG, Manic Fringe/MFNG, Radical Fringe/RFNG) extend O-fucose residues by adding N-acetylglucosamine (GlcNAc) beta-1,3 to fucose. This modification selectively modulates ligand selectivity: LFNG-extended O-fucose on Notch1/2 EGF repeats enhances DLL1/DLL4 binding and suppresses Jagged1/2 binding. This underlies the Fringe-dependent patterning logic in somitogenesis: Fringe expression defines the DLL-responsive domain, creating periodic segmentation boundaries.

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DSL Ligands: Trans-Activation and Cis-Inhibition

Delta-Like (DLL) and Jagged (JAG) Ligands

Five canonical DSL ligands exist in mammals: DLL1, DLL3, DLL4 (Delta-like) and JAG1, JAG2 (Jagged/Serrate). All share:

• •DSL (Delta/Serrate/LAG-2) domain — required for Notch EGF11/12 binding
• •Multiple EGF-like repeats — variable numbers; DLL1: 8 EGF; DLL4: 8 EGF; JAG1: 16 EGF + cysteine-rich domain (CRD)
• •Transmembrane domain and short intracellular tail (no known intrinsic kinase activity)

DLL4: Critical in vascular angiogenesis -- DLL4 expressed on tip cells activates Notch1 in adjacent stalk cells, suppressing stalk cell tip-cell identity (lateral inhibition). DLL4/Notch1 axis controls vessel sprouting patterns.

DLL3: Uniquely unable to activate Notch in trans (cis-only ligand) -- DLL3 lacks key DSL contacts for receptor activation and functions solely as a cis-inhibitor of Notch within the same cell.

JAG1: Activates Notch in trans but is inhibited by Fringe; JAG1/Notch signaling predominates in contexts where Fringe is absent (e.g., mammary gland, bile duct specification).

Endocytosis-Driven Pulling Force Model

Canonical Notch activation requires endocytosis of the ligand by the signal-sending cell. Monoubiquitination of the DLL/JAG intracellular tail by Mindbomb1 (MIB1) or Neuralized (NEURL1/2) E3 ubiquitin ligases targets ligands for epsin-dependent clathrin-mediated endocytosis. As the endocytic vesicle is internalized, the NECD-ligand complex experiences a mechanical pulling force that causes:

1. Conformational change in the NRR (mechanical unfolding of the LNR modules)

2. Exposure of the S2 cleavage site

3. ADAM10 (A Disintegrin and Metalloprotease 10) cleaves at S2

This pulling force model is supported by single-molecule force measurements showing Notch ECD unfolding at ~10 pN -- a force consistent with endocytic vesicle formation. Rigid ligand-receptor bonds (achieved by O-fucose/DSL contacts) are required to transmit force rather than allowing receptor-ligand dissociation.

Trans-Activation vs Cis-Inhibition

DSL ligands on the signal-sending cell activate Notch on the signal-receiving cell (trans-activation). However, ligands expressed on the same cell as the Notch receptor (cis-configuration) inhibit receptor activation (cis-inhibition). The molecular basis of cis-inhibition involves ligand competition for the same EGF11/12 interface that is required for productive trans-activation -- cis-bound ligand prevents ADAM10 from accessing the NRR in the pulling configuration.

This cis-inhibition creates a bistable switch: cells expressing both ligand and receptor favor one state (high Notch activity = high ligand suppressed) or the other (low Notch activity = high ligand production = signal-sending). Lateral inhibition through these mutual repression dynamics generates salt-and-pepper patterns of neuronal differentiation in the neural plate.

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Sequential Proteolysis: S2 (ADAM10) and S3/S4 (Gamma-Secretase)

S2 Cleavage by ADAM10

After NRR unfolding (driven by ligand pulling), ADAM10 (Kuzbanian in Drosophila) cleaves Notch at the S2 site, within the extracellular domain just outside the transmembrane helix (approximately 12 amino acids above the membrane). This releases the large NECD-ligand complex (which is endocytosed by the ligand-presenting cell) and generates the membrane-tethered Notch Extracellular Truncation (NEXT) fragment.

ADAM17 (TACE) can also cleave at S2 but is less specific -- ADAM17-mediated S2 cleavage can occur ligand-independently, generating non-productive NICD. ADAM10 is the physiologically relevant sheddase.

ADAM10 belongs to the ADAMTS superfamily; it contains a prodomain (auto-inhibitory), metalloprotease domain (zinc-dependent; cleaved at HExxH motif), disintegrin domain, cysteine-rich domain, and EGF-like domain. ADAM10 inhibitors used in research include INCB7839 and GI254023X.

S3/S4 Cleavage by Gamma-Secretase

The NEXT fragment is the obligate substrate for gamma-secretase, a multi-subunit intramembrane aspartyl protease complex:

• •Presenilin 1 or 2 (PSEN1/PSEN2): catalytic subunit; nine-pass TM protein; two conserved aspartates (D257, D385 in PSEN1) in TM6 and TM7 form the active site; PSEN mutations cause familial Alzheimer disease by altering APP cleavage
• •Nicastrin (NCSTN): type I TM glycoprotein; ectodomain acts as substrate gatekeeper (steric exclusion of substrates with extracellular domains >~3 residues above TM)
• •APH-1 (Anterior PHarynx-defective 1; APH1A or APH1B): 7-TM scaffold
• •PEN-2 (Presenilin ENhancer 2): 2-TM protein required for PSEN endoproteolysis and activation

Gamma-secretase cleaves NEXT at multiple sites within the TM helix:

• •S3 cleavage (epsilon-equivalent cleavage): releases NICD from the membrane (major transcriptionally active species; Notch1 Val1744 in humans is the primary S3 site)
• •S4 cleavage: generates shorter NICD fragments + Notch beta-amyloid-like fragments (NABe); analogous to APP gamma/beta cleavage

The NICD generated by S3 cleavage is the canonical active species. Val1744 (the N-terminal residue after S3 cleavage) of Notch1 NICD is detected by the Val1744 cleavage-site neoepitope antibody (CST #4147) -- the primary research tool for detecting activated Notch1.

Gamma-secretase inhibitors (GSIs): DAPT (N-[N-(3,5-difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl ester), DBZ (dibenzazepine), LY411575, compound E, avagacestat -- all competitively inhibit PSEN active site; block S3/S4 cleavage; prevent NICD release; suppress Notch target gene expression. DAPT and DBZ are the most widely used research tools. GSIs also block gamma-secretase cleavage of other substrates (APP, ErbB4, CD44, N-cadherin) -- important confound for specificity.

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NICD Nuclear Function: CSL Complex and Transcriptional Activation

CSL Repressor Complex (WNT-OFF State Analogy)

In the absence of Notch signaling, the transcription factor CSL (CBF1/RBPJ in mammals; Suppressor of Hairless in Drosophila; LAG-1 in C. elegans; collectively CSL or RBPJ) binds Notch target gene promoters and recruits transcriptional co-repressors:

• •SHARP/MINT/SPEN: contains NLS + three RNA-recognition motifs + an SPOC domain; recruits NCoR/SMRT co-repressor complex + HDAC1/2/3
• •SKIP: SKI-interacting protein; bridges CSL to SMRT/NCoR
• •KyoT2: LIM domain protein; competes with NICD for CSL binding

CSL contacts the major groove of the Notch response element (NRE: YGTGRGAA consensus) through a beta-trefoil fold (BTD domain) and beta-strand (NTD domain). CSL forms a stable core complex that recognizes NRE motifs at paired sites (SPS: sequence-paired sites) 15-17 bp apart in opposite orientations.

NICD-MAML-CSL Ternary Activator Complex

Nuclear NICD directly binds CSL through two interfaces:

1. RAM domain: high-affinity (Kd ~15 nM) contact with CSL BTD; unstructured RAM wraps around the CSL hydrophobic groove

2. ANK repeats: lower-affinity contact; cooperative with RAM; positions MAML

Mastermind-Like (MAML1, MAML2, MAML3) co-activators are essential for Notch transcriptional activation. MAML1 contains an N-terminal basic helix that contacts both the NICD ANK repeats and CSL simultaneously -- MAML cannot bind NICD or CSL alone; only the pre-formed NICD-CSL binary complex creates the MAML binding surface. The NICD-CSL-MAML ternary complex recruits:

• •p300/CBP: HAT co-activators that acetylate surrounding histones (H3K27ac marking active enhancers)
• •CDK8/Mediator: MAML C-terminus interacts with Mediator, driving RNA Pol II pause release
• •SKIP: repositioned from repressor function to co-activator in NICD-containing complexes

NICD degradation: MAML1 C-terminus also recruits CDK8, which phosphorylates the NICD PEST domain, creating a phosphodegron recognized by Fbxw7 (FBXW7/SEL-10/CDC4), the substrate receptor of the SCF^Fbxw7^ E3 ubiquitin ligase. This creates a built-in timer: MAML recruits both the co-activator (p300) and the destruction machinery (CDK8 -> Fbxw7). FBXW7 is mutated in ~30% of T-ALL and multiple solid tumors, stabilizing NICD and prolonging Notch target gene activation.

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Key Notch Target Genes

HES1 (Hairy and Enhancer of Split 1): bHLH-O transcriptional repressor; contains WRPW tetrapeptide for Groucho/TLE recruitment; represses proneural genes (ATOH1, NEUROG1/2, ASCL1); oscillates with ~2 h period in presomitic mesoderm (Notch oscillation clock); primary readout of Notch activity by qPCR/Western

HES5: bHLH-O repressor; redundant with HES1 in neural contexts; CSL-dependent, parallel to HES1

HEY1, HEY2, HEYL (Hairy/Enhancer of split-related with YRPW motif): bHLH-O repressors; contain YRPW instead of WRPW; HEY1 represses cardiac mesoderm and vascular smooth muscle differentiation; HEY2 critical in cardiac trabeculation

MYC: Direct Notch1 target in T-ALL; Notch1 activates MYC through a long-range enhancer ~1.4 Mb downstream of the MYC locus ("N-Me" enhancer); NICD1 occupies this enhancer by ChIP-seq; GSI treatment reduces MYC mRNA and protein within 4-8 h in NOTCH1-mutant T-ALL cell lines

NRARP (Notch-Regulated Ankyrin Repeat Protein): ANK-repeat protein that competes with MAML for NICD-CSL complex binding; NRARP is itself a Notch target gene -- constituting a direct negative feedback loop. NRARP also activates WNT/beta-catenin by stabilizing Axin1/DVL, interconnecting Notch and WNT at the feedback level.

HES1 oscillation: Notch-driven HES1 expression auto-represses itself with a ~30 min delay (HES1 represses its own promoter + HES1 protein has a ~22 min half-life due to polyubiquitination at K14). This generates the segmentation clock oscillation in presomitic mesoderm cells, coordinating somite boundary formation.

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Post-Translational Regulation of Notch

NUMB/NUMBL: endocytic adaptor proteins that interact with Notch intracellular domain (PTB domain binds NPXY-like motifs); promote Notch endocytosis and lysosomal degradation; asymmetrically segregated during cell division to create daughter cell Notch activity asymmetry -- the molecular basis of asymmetric cell division.

Deltex (DTX1-4): RING-H2 domain E3 ubiquitin ligases that promote K63-linked ubiquitination of NICD, directing it for endosomal trafficking vs proteasomal degradation; Deltex and Numb compete for Notch fate regulation.

WWP2 / Itch (ITCH): HECT domain E3 ligase; ubiquitinates NICD at multiple lysines, targeting for proteasomal degradation; opposes Notch activation.

Fringe (LFNG/MFNG/RFNG): as described -- EGF repeat O-GlcNAc extension modulating ligand selectivity.

O-glucose modification: POGLUT1 (Protein O-Glucosyltransferase 1) attaches O-glucose to a distinct consensus on Notch EGF repeats; O-glucose extensions (xylose additions by GXYLT1/2) regulate Notch surface expression and stability.

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Research Tools for Notch Pathway Investigation

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Experimental Protocols

Protocol 1: Gamma-Secretase Inhibitor Western Blot Panel (NICD1 / HES1 / HES5)

Validates GSI on-target activity and establishes dose-response for Notch pathway suppression.

Reagents: DAPT (10 mM stock in DMSO), DBZ (1 mM stock in DMSO), Compound E (1 mM stock), RIPA lysis buffer + PhosSTOP + protease inhibitor cocktail, antibodies: anti-cleaved Notch1/NICD1 (CST #4147 Val1744 neoepitope), anti-HES1 (CST #11988), anti-Notch1 total (CST #3608), anti-beta-actin

Protocol:

1. Seed T-ALL cells (HPB-ALL, DND-41, or Notch1-mutant cell lines) or adherent cells (NIH3T3 + plate-bound DLL4) at 5x10^5/ml

2. Treat with DAPT (0.1, 1, 5, 25 uM), DBZ (1, 10, 30, 100 nM), or DMSO vehicle; 24 h incubation

3. Harvest cells; lyse in RIPA buffer (150 ul per 5x10^6 cells); rotate 20 min 4 degrees C; centrifuge 16,000xg 15 min

4. Western blot: 30-50 ug protein per lane; use 6-8% gel for Notch1 full-length (~300 kDa) and NICD1 (~120 kDa); transfer at 30V overnight for high-MW proteins

5. Primary antibodies: anti-NICD1-Val1744 (1:1000; detects only S3-cleaved fragment, not full-length or NEXT); anti-HES1 (1:1000; 28 kDa); anti-total-Notch1 (1:1000); anti-actin

6. Dose-response: plot NICD1 band intensity vs [inhibitor]; calculate IC50 by 4-parameter logistic fit; HES1 suppression typically parallels NICD1 with 4-8 h delay (protein half-life ~22 min, so HES1 depletion is rapid after NICD1 loss)

7. Validation: NICD1 band should appear only in vehicle-treated/Notch-active cells; GSI abolishes band; total Notch1 increases upon GSI (S3-cleaved forms accumulate as NEXT when gamma-secretase is blocked)

Protocol 2: CSL Reporter (pGa981-6 / 4xCSL-Luc) Notch Transcriptional Assay

Quantifies Notch-dependent transcription in a cell-based luciferase reporter system.

Reagents: pGa981-6 reporter (6x CSL binding sites driving firefly luciferase; Addgene), pRL-TK (Renilla control), plate-bound DLL4-Fc or JAG1-Fc (5 ug/ml in PBS, 2 h RT coating, wash), DAPT control, lipofectamine 3000, Dual-Glo system

Protocol:

1. Coat 24-well plates with DLL4-Fc or JAG1-Fc (5 ug/ml in PBS 200 ul/well; 2 h RT; aspirate; wash PBS; proceed immediately or air-dry for storage)

2. Transfect NIH3T3 or HEK293T cells: pGa981-6 (200 ng) + pRL-TK (20 ng); lipofectamine 3000; 24 h

3. Transfer transfected cells to DLL4-Fc-coated or uncoated wells; treat +/- DAPT (25 uM) or DBZ (100 nM); 24 h

4. Dual-Glo luciferase: measure firefly (Notch transcriptional output) and Renilla (transfection normalization)

5. Fold-activation: (firefly/Renilla in DLL4-coated) / (firefly/Renilla in uncoated); DAPT-treated DLL4 coated = background control

6. Dominant-negative MAML (dnMAML) overexpression: transfect dnMAML-GFP construct; abolishes reporter signal even in DLL4-coated wells; validates MAML-dependency of reporter

7. Reciprocal experiment: overexpress NICD1 (truncated intracellular domain construct) -- constitutive reporter activation independent of ligand/receptor

Protocol 3: HES1 mRNA Kinetics by RT-qPCR (Oscillation vs Sustained Activation)

HES1 mRNA oscillates with a ~2 h period in presomitic mesoderm cells but shows sustained elevation in Notch-hyperactive cancer cells. This protocol distinguishes oscillatory from sustained activation.

Reagents: RNeasy micro kit, iScript cDNA synthesis, SYBR Green qPCR mix, HES1 primers (fwd: 5-GCAGACATTCTGGAAATGACAGTG-3; rev: 5-GGAGCTATCTTTCTTAAGTGCATCG-3), ACTB primers, DAPT for inhibitor experiments

Protocol:

1. Synchronize oscillating cells (presomitic mesoderm-like cell lines or primary cultures): serum shock (50% horse serum, 2 h) then return to low-serum medium

2. Collect cells at 30-min intervals over 6 h; extract RNA immediately (snap-freeze in RLT buffer)

3. Reverse transcribe; qPCR for HES1 and ACTB; calculate delta-CT; plot HES1 mRNA vs time

4. Oscillating cells: sinusoidal HES1 pattern with ~2 h period; DAPT addition flattens oscillations (Notch-dependent clock)

5. Sustained activation experiment: T-ALL or MDA-MB-231 (Notch-active) cells; collect hourly; HES1 mRNA elevated but non-oscillating

6. GSI dose-response on T-ALL: 0, 0.1, 1, 10 uM DAPT; 6 h treatment; RT-qPCR for HES1, HES5, MYC, NRARP; IC50 for HES1 mRNA typically 0.5-2 uM DAPT

Protocol 4: Notch1 Surface Expression and ADAM10 Shedding by Flow Cytometry

Measures Notch1 surface levels and ADAM10-dependent S2 shedding using antibodies against the extracellular domain.

Reagents: Anti-Notch1 ECD antibody (clone bTAN20 or CST #3608; fluorochrome-conjugated or secondary detection), anti-ADAM10 antibody, INCB7839 (ADAM10i, 1-10 uM), recombinant DLL4-Fc (10 ug/ml), FACS buffer (PBS + 2% BSA + 2 mM EDTA), live/dead discriminator (DAPI or viability dye)

Protocol:

1. Harvest cells (non-enzymatic dissociation for surface epitope preservation); wash 2x cold FACS buffer

2. Surface staining: incubate with anti-Notch1-ECD primary antibody (1:100) 30 min 4 degrees C; wash; secondary if needed; live/dead discriminator

3. Acquire on flow cytometer; gate live singlets; report Notch1 ECD MFI

4. DLL4 competition experiment: incubate cells with DLL4-Fc (10 ug/ml) 30 min 4 degrees C before surface Notch1 staining -- DLL4-Fc competes with anti-Notch1-ECD for EGF11-12 epitopes; confirm DLL4 binding reduces Notch1 ECD signal

5. ADAM10 shedding: treat cells with INCB7839 (1-10 uM, 2 h); measure Notch1 ECD -- ADAM10 inhibition prevents S2 cleavage, causing ECD accumulation (higher MFI); soluble NECD shed into conditioned medium measured by anti-ECD ELISA

6. Fringe modulation: overexpress LFNG; measure DLL4-Fc binding (ligand binding assay using DLL4-Fc + secondary anti-Fc-PE); LFNG increases DLL4-Fc binding to Notch1/2; JAG1-Fc binding decreases

Protocol 5: In Vitro NICD1-CSL-MAML1 Ternary Complex Assembly (TR-FRET or Pull-Down)

Biochemical assay to screen compounds that disrupt the transcriptional activation complex.

Reagents: Recombinant His-NICD1 (RAM + ANK domain, purified from E. coli or baculovirus), recombinant GST-CSL/RBPJ, recombinant MBP-MAML1 (N-terminal helix, aa 1-75), biotinylated NRE DNA duplex (YGTGRGAA), Tb-anti-His donor (Cisbio), d2-anti-GST or anti-MBP acceptor (Cisbio), TR-FRET plate reader

TR-FRET Protocol:

1. In assay buffer (50 mM HEPES pH 7.4, 150 mM NaCl, 0.05% Tween-20, 0.1% BSA): combine His-NICD1 (10 nM), GST-CSL (20 nM), biotinylated NRE DNA (50 nM); pre-incubate 30 min RT

2. Add MBP-MAML1 (30 nM); incubate 60 min RT for ternary complex formation

3. Add Tb-anti-His donor + d2-anti-GST acceptor (each at 1-2 nM); 60 min RT; read TR-FRET (ex 337 nm; em 665 nm / 620 nm ratio)

4. Ternary complex TR-FRET signal: high ratio when NICD1-CSL-MAML1 assembled; disruption by SAHM1 (50-1000 nM) or dnMAML peptides reduces ratio

5. Compound screening: pre-incubate compound (0.1-100 uM) with NICD1-CSL binary; add MAML1; measure TR-FRET inhibition; Z-prime factor (>0.5) validates assay quality for HTS

6. Pull-down alternative: His-NICD1 + GST-CSL + biotinylated-MAML1-N-helix; pull down on Ni-NTA beads; elute; detect biotin-MAML1 by streptavidin-HRP blot; compound disruption of pull-down indicates MAML displacement

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Notch in Disease Contexts

T-ALL: NOTCH1 Activating Mutations

Activating NOTCH1 mutations occur in >50% of T-ALL cases. Two major mutation classes:

1. HD domain mutations (NOTCH1 L1596P, A1580T, etc.): destabilize the NRR auto-inhibitory conformation; allow ligand-independent ADAM10/gamma-secretase cleavage; typically moderate gain-of-function

2. PEST domain mutations (frameshifts, truncations removing PEST): prevent Fbxw7-mediated NICD1 degradation; prolong NICD1 half-life; typically strong gain-of-function

Some T-ALL tumors harbor both HD + PEST mutations, creating maximal NICD1 accumulation. HPB-ALL, DND-41, CCRF-CEM, Jurkat (weak) are commonly used NOTCH1-mutant T-ALL cell line models. GSIs suppress MYC and HES1 in these cells; FBXW7-mutant lines show residual NICD1 even after GSI (PEST degradation pathway lost).

FBXW7 as Tumor Suppressor

FBXW7 (F-box and WD repeat domain containing 7) is mutated in ~30% of T-ALL, ~10% of CRC, and multiple other cancers. FBXW7 recognizes phospho-PEST degrons in substrates including NICD1 (T2514/T2512 phosphorylated by CDK8), MYC (T58/S62 phosphorylated by GSK3beta), cyclin E, c-JUN, and MCL-1. Loss of FBXW7 stabilizes all these oncoproteins simultaneously, contributing to broad oncogenic reprogramming. In Notch contexts, FBXW7 loss creates persistently elevated NICD1 that is partially GSI-insensitive (because even if S3 cleavage is reduced, existing NICD1 is not degraded).

Triple-Negative Breast Cancer: JAG1/Notch3 Axis

JAG1-Notch3 signaling maintains cancer stem cell (CSC) properties in triple-negative breast cancer (TNBC) -- JAG1-high cancer cells activate Notch3 in neighboring cells, promoting CD44+/CD24- CSC marker expression, mammosphere formation, and chemotherapy resistance. GSI treatment or anti-JAG1 antibodies reduce mammosphere-forming efficiency in research models.

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