Wnt Signalling Flashcards
Wnt is an amalgam of what to genes?
Wingless (drosophila)
int (vertebrate oncogene which encodes the mouse homologue of wingless
When were the first wingless alleles discovered?
in the 70s
Loss of function alleles of wingless were found to show what in drosophila?
Segment polarity disruption
Loss of wingless or hedgehog shows what phenotype?
A lawn of denticles over the whole embryo
How do wingless and hedgehog control each other in segments?
Autoregulatory loop
Hh directly upregulates Wg transcription
Wg controls the expression of engrailed which controls expression of Hh
Describe briefly how the Wnt signalling ligand is produced
1) The protein is transcribed
2) The Wnt signal is modified
3) The protein is presented due to another protein called wintless
4) HSPGs are important for the signal to form
Where is the wnt ligand modified?
Cys77 = palmitoylation (acly transferase porcupine) ser209 = palmitoleic acid (acyl transferase porcupine)
What is the role of wntless?
Required for transport of wnt to the plasma membrane ans possibly also for release and appropriate presentation of Wnt responding to cells
What are heparan sulfate proteoglcans used for?
Diffusion away from the sending cell
How was it shown that juxtacrine signalling mediated via cell-cell contact is sufficient in Wnt signalling?
Wnt has been modified so that it is unable to diffuse away from the signalling cell
What are the human homologues for:
a) Frizzled (1-4)
b) Arrow
c) Armadillo
a) FrZ (1-10)
b) LRP5 and 6
c) beta Catenin
Frizzled, arrow and LRP5 are what?
Receptors for Wnt
Armadillo and beta catenin are what?
Nucleofactors
What does LRP stand for?
LDL related protein
Where does Wnt bind to on Fz?
The N terminal extension known as the cysteine rich domain (CRD)
How many times does LRP/arrow pass through the membrane?
Once - signle pass transmembrane protein
When is Wnt signalling initiated?
When the two types of receptors eg Fz and LRP5 are bought together
Name an important Wnt Inhibitor
Dikkopf1
Overexpression of Dikkopf1 causes what?
Downregulation of the Wnt signal
How does Dikkopf1 work?
It binds to LRP/arrow and couples it to kremen - another transmembrane protein
This causes the internalisation of arrow/LRP
What happens to betacatenin in the absence of Wnt?
It is at a low level because it is being continuously broken down
Which proteins make up the betacatenin destruction complex?
Axin, APC, CK1 and GSK3B
What happens to betacatenin when it binds to the destruction complex in the absence of Wnt?
1) Betacatenin is phosporylated by Ck1 and then GSK3B
2) As a result it is recognised by Slimb = ubiquitination
3) Beta catenin is degraded by proteosomes
What happens to beta catenin in the presence of Wnt signalling?
1) Arrow and frizzled are bought together so dishevelled (Dsh) is recruited and phosphorylated
2) Dsh binds axin
3) GSK3B phosphorylates Arrow, creating more binding sites for arrow
4) Slimb is lost from the destruction compex
5) Beta catenin is phosphoryated but is not ubiquniated
Normally groucho blocks the transcription of what?
TCF
What is required for Beta catenin to be recognised by an ubiquitin ligase complex?
Phosphorylation by both complexes - CK1 alpha and GSK3
If the CK1 serine recognition site is phosphorylated on beta catenin/armadillo what does it become?
A recognition site for GSK3
When does B-TrCP/Slimb bind to beta catenin?
Only when GSK3 has phosphorylated the the 3rd and 4th phosphorylation sited
What is grocuho?
A transcriptional repressor bound to TCF
How does groucho act?
Recruits histone deacetylases from the histones causing tighter DNA packaging
Give an example of a chromatin remodelling factor
Brg1
What are pygopus and legless?
Identified in drosophila
Important co-factors for transcription
List three negative regulators of Wnt signalling
Dkk1, Dkk4 and Axin2
List examples of genes which are used in the non-conical Wnt pathway
Vang11/2, Celsr, pickle, inversin, Rho and Rac
How can Wnt5a inhibit canonical Wnt signalling?
Through ror2
Mutations in Vang12 in fish show what phenotype?
Convergent extension does not work well so the somites are squashed
List two roles of canonical wnt in drosophila
1) Segmentation
2) Required for patterning and outgrowth of the DV boundary of the wing
What is the role of Wnt11 in vertebrate embryogenesis?
Beta catenin makes the organiser and induces dorsal
Wnt 8 and 3 induce what?
Posterior/ventral fates
Zygotic loss of Axin 1 causes what?
Wnt activation leading to posteriorisation of the anterior brain
This causes the loss of eye vesicles
Why is Wnt signalling important in the gut?
Stem cells divide to make new cells of the gut
Wnt signalling loss would cause stem cell loss
Ectopic Wnt signalling can occur due to the loss of which gene?
APC
What is familial adenomatous polyposis?
Heterozygouse for the APC gene
Accidental loss of remaining APC gene
Wnt signalling is activated and as a result the cell with the mutation will always stay as a stem cell, leading to polyps in their colon/rectum => cancers
How often is it that colon cancer is caused of a gof in Wnt signalling
90% of the time
Axin LOF leads to what?
hepatocellular carcinoma
What is Wnt3-tetra-amelia?
Rare genetic disease where babies are born with no limbs
How can bone density be increases as a result of Wnt singalling?
LRP5 point mutation causes an insensitivity to Dkk causing GOF in the pathway
Axin 2 mutations can cause what?
Severe tooth agenesis or oligodontia (multiple missing teeth)
