Wnt signalling Flashcards
What is the role of Wg in drosophila
Complete loss of function alleles were found to disrupt segment polarity of the embryo
It controls the expression of engrailed which is a transcription factor that regulates Hh expression - Hh expression upregulates Wg so they act to maintain eachothers expression in an autoregulatory loop
What is the phenotype for a wingless mutant
The loss of wingless mimics the hedgehog mutant phenotype. A continuous lawn of denticles.
What two modifications are added to the Wnt protein and what mediates the addition
Palmitoylation and a palmitoleic acid modification of serine 209 mediated by acyl transferase
What affects do the modifications made to the Wnt protein have
Make it hydrophobic so in order to be released from the sending cell it may form multimers or be loaded onto lypoprotein particles
What is the role of HSPGs in Wnt signalling
required for the diffusion of Wnt away from the sending cell
What was the effect of making a membrane bound form of the Wnt protein in drosophila
Most effects of the Wnt signalling pathway were still elicited suggesting that juxtacrine signalling is sufficient in many cases
What proteins make up the Wnt receptor
Frizzled and LRP/Arrow - Wnt binds to the N terminal extension of frizzled. Wnt also comes into contact with LRP/arrow so Wnt may serve to bring frizzled and LRP together
What is the most important extracellular Wnt inhibitor and when is it used experimentally
Dickkopf 1 - its overexpression is often used where the level of Wnt signalling needs to be downregulated. Dickkopf 1 acts to reduce Wnt signalling by reducing the amount of LRP at the cell membrane
Dkk couples LRP to kremen - this binding promotes the internalisation of LRP
Outline the signalling cascade when Wnt is not present
Beta catenin is phosphorylated by a destruction complex comprised of Axin (a scaffold protein), APC, GSK3beta and Ckla. APC is a large multifunctional protein that binds beta catenin.
The phosphorylated form of B catenin is recognised by Slimb which is associated with the destruction complex
This causes B catenin to become ubiquitinated and degraded by the proteasome.
In the absence of nuclear B catenin TCFs are bound to the promoter of Wnt responsive genes but are associated with Groucho, which is a transcriptional repressor, in this way expression of Wnt target genes is inhibited
What happens when Wnt binds to the receptors
Dishevelled is recruited by frizzled and is phosphorylated.
In addition, LRP/Arrow is phosphorylated and recruits axin to the receptor
Crucially, Slimb is thought to be lost from the complex - this inactivates the destruction complex
Beta catenin therefore accumulates in the cytoplasm and translocates to the nucleus
Beta catenin displaces Groucho from TCF DNA binding proteins
In combination with some additional transcriptional regulators it will activate target gene transcription
How is Beta catenin degraded
First phosphorylation by Ckla which primes phosphorylation by GSK3
Phosphorylation by both kinases is required for recognition by ubiquitin and subsequent degradation
Which phosphorylation sites must be phosphorylated by GSK3 before ubiquitin/Slimb binds to Beta catenin
3rd and 4th
What occurs in the nucleus when there is no Wnt
TCF acts as a repressor by binding to groucho. The repressing effect is mediated by histone deacetylase, which are thought to make DNA refractive to transcriptional activation
What occurs when Beta catenin is present in the nucleus
Recruitment of the histone acetylase CBP and BRG-1 which may induce chromatin remodelling that favours target gene transcription
What is the role of canonical Wnt in drosophila
Segmentation, also expressed at D/V boundary of the wing required for its patterning and outgrowth
