Morphogens Flashcards
What must a morphogen do?
Induce different outputs at different concentrations
How would you test to show that a morphogen is instructive and not permissive (Test 1)
Provide a second source.
If it is instructive then a mirror image is produced.
If it a permissive signal then even with a second source there would be no effect on the fate of each cell
Example - Ectopic Shh expression creates a mirror of digits in the chick limb
How would you test to show that a morphogen is instructive and not permissive (Test 2)
Provide a uniform concentration of the signal
An instructive signal will cause only one cell type to be produced
A permissive signal will have no effect
How would you test to show a morphogen acts directly at a distance
A signalling cell will be responsible for patterning the whole field of cells if it is a morphogen. If it is not it will use the bucket brigade mechanism - each signal induces the next cell to produce another
How would you test to show a morphogen acts directly at a distance (Test one)
Make the proposed morphogen a juxtacrine by genetic engineering (add a transmembrane domain)
A morphogen would cease to work
If it was acting via the bucket brigade it would have no effect
How would you test to show a morphogen acts directly at a distance (Test two)
Make a genetic mosaic that lacks the receptor for the signal in one of the cells
The morphogen would mimic the cell that receives no signal in the cell that has the lack of a receptor
The bucket brigade would be unaffected unless the receptor was removed from the cell immediately next to the signalling cell.
Does passive diffusion establish a morphogen gradient
Passive diffusion would only generate a shallow gradient
How is a steep morphogen gradient generated
Restricted diffusion - Binding of signalling molecules to molecules in the ECM and high concentrations of receptor
Also caused by rapid degradation of the signal in the extracellular space.
What are HSPGs and what is their role in regulating morphogen diffusion
Known to bind to many ligands. Sometimes called co-receptors
They regulate morphogen diffusion by sequestering or slowing diffusion or facilitating diffusion
What is planar transcytosis
A pit forms in the cell membrane and engulfs the morphogen in a vesicle. Repeated cycles of endocytosis and resecretion allows certain morphogens to travel through cells in a tissue
What is the evidence for transcytosis in `Dpp signalling
Antibody staining shows that Dpp is found in vesicles and mutations that block vesicle formation cause Dpp to act as a juxtacrine.
What is the transcription readout model
Higher concentration of morphogen often results in a higher concentration of an activated transcription factor
What is the role of Bicoid
Bicoid is a morphogen and a transcription factor
bicoid mRNA is localised at the anterior of the egg and is translated into protein during early embryogenesis.
Bicoid protein then diffuses through the cytoplasm and accumulates in nuclei of the syncytial blastoderm generating a concentration gradient
How is transcription factor interpreted at the DNA level
Enhancers that regulate different cell types have differing affinities for TFs
Even though they see the same amount of the signal, only high affinity enhancers can bind enough TF to activate gene expression
If there is a high conc of TF both enhancers are activated - why does the low affinity enhancer cell not have high affinity gene expression? - Because the low affinity cell fate genes encode a repressor - this is crosstalk
