L10: Wnt Flashcards
(11 cards)
wnt
wnt-1, originally called int-1, gene activated by MMTV insertion, but renamed after being found homologous to the segment polarity gene, wingless, in Drosophila.
Prototype of a large family encoding secreted proteins important for pattern formation in the embryo.
Activates a signalling pathway important in colorectal cancer in which the APC tumour suppressor also participates.
wnt signalling pathways: wntbeta-catenin pathway: cell proliferation, survival and fate
jnk athwayL cell shape, orientation and migration
wnt ligand secretion
Wnt ligand secretion requires porcupine in the er and wntless in the golgi
Porcupine (PORC) is an acyltransferase that adds a palmitoleic acid moiety to the Wnt ligand in the ER.
This makes Wnts hydrophobic and they do not diffuse that far.
Wntless is needed to transport Wnt ligands through the Golgi.
Wntless is dedicated to the Wnt ligands i.e. it does not transport other proteins.
Wntless helps chaperone the wnt ligand and make sure it is secreted. If porc not functioning no appropriate post trans mod of wnt. Wnt are hydrophobic and very sticky, tend to diffuse across a few cells maximum. ?? proect over many cell distances and can project wnt to target?
Wntless only chaperone wnt ligand nothing else.
wnt beta catenin
🔷 What is Wnt/β-catenin Signalling?
It’s a signalling pathway that regulates stem cell renewal, proliferation, and cell fate. In the colon, it maintains stem cells in the crypts. Misregulation leads to diseases like colon cancer.
🔷 Pathway OFF (No Wnt Signal)
Wnt ligand is absent
Destruction complex is active:
Made up of: APC, Axin, GSK3β, CK1
It phosphorylates β-catenin at specific sites (Ser33, Ser37, Thr41, Ser45)
hTRCP (an E3 ligase) recognizes phosphorylated β-catenin → tags it for degradation by the proteasome
Low β-catenin in the cytoplasm → can’t enter nucleus
Tcf/Lef transcription factors remain inactive → no Wnt target gene expression
🔷 Pathway ON (Wnt Present)
Wnt ligand binds to its receptors:
Frizzled and LRP5/6
This recruits and activates Dishevelled (Dsh), which inhibits the destruction complex
β-catenin is stabilized → it accumulates in the cytoplasm and moves into the nucleus
In the nucleus, β-catenin binds Tcf/Lef transcription factors
This activates transcription of Wnt target genes, including:
c-Myc, Cyclin D1 → promote proliferation
MMP7, VEGF, Lgr5, etc.
🔷 Regulators and Modulators
R-spondin: enhances Wnt signalling by inhibiting Rnf43/Znrf3
Rnf43/Znrf3: E3 ligases that reduce Wnt signalling by degrading Frizzled receptors
Dkk1: secreted protein that inhibits Wnt by blocking LRP5/6
🔷 β-Catenin’s Dual Role
Intracellular Signalling (as above in Wnt pathway)
Cell Adhesion:
At the membrane, β-catenin binds E-cadherin and α-catenin as part of adherens junctions
Important for cell-cell adhesion
🔷 Why It Matters in Cancer
APC mutations or mutations in β-catenin prevent its degradation → constant Wnt signalling
Leads to excessive proliferation, stem cell expansion, and tumor growth
wnt transcriptional target genes
wnt transcriptional target genes include negative regulators of the Wnt pathway.
Rnf43 & Znrf3 promote Wnt receptor degradation
R-spondin together with Lgr4/5/6 removes Rnf43/Znrf3 away from the receptors
wnt mutations in cancer
Wnt mutations & Cancer
Note
Contribution of de-regulated canonical Wnt signalling to different cancers
differences in the frequency of which components of the pathway are mutated in different cancers
Colon Cancer Risk Factors: De-regulation of Wnt signalling is the genetic hallmark of colorectal cancers
APC mutations
- These were first identified in hereditary form of colon cancer -
Familiar adenomatous polyposis (FAP) - APC mutations occur in 80% of sporadic colon cancers
- APC can’t bind β-catenin and axin - therefore β-catenin released from the destruction complex
- Lose the nuclear export signal and therefore, can’t shuttle β-catenin out of the nucleus and deactivate the pathway
- Get hyperplasia of colon crypt cells and the formation of polyps
yperproliferation –
Canonical Wnt active here: Mutations in the APC gene (a key regulator of the Wnt pathway) lead to constitutive Wnt activation. This causes increased proliferation of colonic epithelial cells — the first step in tumor initiation.
Adenomatous Polyps (Early Adenoma) –
Continued Wnt signaling drives uncontrolled growth, resulting in the formation of benign adenomatous polyps.
Severe Dysplasia (Late Adenoma) –
A second hit (e.g., mutations in KRAS) promotes further growth and abnormal cellular architecture, pushing the lesion toward malignancy.
Adenocarcinoma (Invasive Cancer) –
Additional mutations (e.g., in TP53, SMAD4) represent further “hits” that disrupt cell cycle regulation and apoptosis, enabling invasion and malignancy.
Cancer Progression –
Multiple accumulated mutations drive full malignant transformation and potential metastasis.
wnt normal colon
Wnt Activity in the Normal Colon
Active in stem cells and transit-amplifying cells at the crypt base.
Lgr5+ cells: Wnt-responsive stem cells responsible for replenishing the epithelium.
Tcf4: Transcription factor mediating Wnt target gene expression.
β-catenin: Central effector; stabilized when Wnt is active → translocates to the nucleus to activate gene transcription.
Dkk1 and SFRPs: Secreted Wnt antagonists; inhibit Wnt signalling.
Axin & APC: Tumor suppressors in the β-catenin destruction complex.
hTRCP: Targets phosphorylated β-catenin for degradation.
Rnf43: E3 ligase, limits Wnt receptor signalling (negative feedback).
beta-catenin target genes
C-myc promotes cell proliferation
Cyclin D1 promotes cell proliferation
MMP7 (metalloprotease matrilysin).
Ephrin receptors EphB2 and EphB3 and ligand, ephrin B1. These factors control proliferation and migration of cells towards top of villus.
Lgr5 and Rnf43, stem cell components that are part of the canonical Wnt signalling pathway
Vegf – promotes angiogenesis
Epithelial to Mesenchymal Transformation markers, such as N-cadherin, Vimentin, Snail-1,
ephrin-b
Ephrin-EphB Signaling in the Intestinal Villi
Function: Regulates cell positioning, migration, and proliferation.
EphB2/EphB3 receptors and ephrinB1 ligand:
Keep proliferative stem/progenitor cells confined to the crypt base.
Guide cells to migrate upwards toward the villus tip.
Disruption:
Loss or dysregulation causes mis-migration of cells.
Leads to abnormal accumulation of undifferentiated, proliferative cells—contributing to tumor initiation.
apc at mcr
APC and the Mutation Cluster Region (MCR)
APC gene: Tumor suppressor with multiple roles in:
Regulating β-catenin degradation.
Maintaining cytoskeletal integrity.
Chromosome segregation and migration.
MCR: Region in APC where truncating or missense mutations frequently occur.
Effect of APC mutations:
Early driver of colorectal cancer.
Causes clonal expansion of mutated stem cells.
Leads to Wnt pathway activation, increased β-catenin, and genetic instability.
Drives polyp formation when occurring in stem cells.
🔷 Comparisons with Other Wnt Pathway Mutations
Axin and β-catenin mutations:
Less common and typically less severe than APC mutations.
Still can activate Wnt signaling but do not have the same breadth of impact on cellular function.
Key Concept: APC loss in stem cells is a critical initiating event in colorectal tumorigenesis, amplified by disrupted Ephrin-EphB signaling that allows misplaced, proliferative cells to persist and expand abnormally.
