Morphogens Flashcards
What is a morphogen?
A SOLUBLE, SECRETED molecule that acts at a DISTANCE to specify the FATES of cells
How does a morphogen specify more than one type of cell?
Through a concentration gradient
Describe morphogen gradients
- Once cell produces one instructive molecule
- Cells respond to the morphogen in a concentration-dependant manner
- Cells that don’t receive any morphogen (too far away) default to a specific form
- Cells close to the source of the morphogen - see higher concentration
- If the cell sees morphogen concentration above a certain threshold - the cell will develop into a certain fate
- Many thresholds of morphogen, decreasing in concentration - specify different fates
What happens if there is an ectopic source of morphogen?
There is a mirror image of pattern formation
Are all molecules that are involved with patterning morphogens?
No
What are the possible function of a diffusible ligand?
What type of signal is this?
Tells the cell to assume a fate they already know, by permitting the cell to respond to another source of information
‘Permissive signal’
Are permissive signals morphogens?
Why?
No
Permissive signals do not provide information - they just act as a ‘switch’
What 2 things MUST a molecule do to be a morphogen?
1) Induce different outputs at different concentrations
2) Act directly at a distance
If a patterning molecule doesn’t do this - likely to be permissive
How can we distinguish between a permissive signal and a morphogen (instructive signal)
Using the information ‘a morphogen must induce different outputs at different concentrations’
Using 2 tests:
1) Provide a second source of the morphogen
2) Provide the signal at uniform concentrations
Describe what happens if the is a second concentration of a true morphogen?
- Mirror image of cell fates
- Loss of fates that respond to LOW concentrations of morphogen
Describe what happens if the is a second concentration of a permissive signal?
Cells will still assume their original fate
How was Shh discovered in the wing bud?
What does this show?
Through providing a second source of the morphogen:
- Ectopically express shh in the anterior of the wing bud (usually in the posterior of the wing)
- Lead to mirror image of the digits
Shows shh to be a morphogen that patterns the position and shape of the digits
What happens if you provide an instructive signal at a uniform concentration?
Permissive signal?
Instructive signal - All cells will adopt the SAME fate (all see the same concentration of morphogen)
Permissive signal - no difference
What is another name for a morphogen?
Instructive signal
What is the ‘bucket brigade’?
What molecule is this method not used for?
Indirect action:
Cell that sees the highest concentration - assumes is fate and produces a new molecule that acts on the next cell to specify the fate of that cell and so on
In a cascade
NOT used for morphogens
How do morphogens act at a distance?
Directly:
- Released from one cell
- Patterns following cells in a concentration-dependant manner
How do we distinguish if a molecule is acting over distance directly or indirectly (bucket brigade)?
2 tests:
1) Use genetic engineering to make the proposed morphogen juxtacrine (add transmembrane domain)
2) Use a genetic mosaic organism that lacks the receptor for the proposed morphogen
What happens if the signalling molecule is juxtacrine, if the molecule was a morphogen?
- Molecule can no longer diffuse away from its source
- If true morphogen, only the cell immediately in contact with the cell producing the morphogen will assume its fate
- The rest of the cells will assume the default fate (as if not seeing any morphogen)
- Loss of patterning
What happens if the signalling molecule is juxtacrine, if the molecule signalling using a bucket brigade system?
- Patterning not affected
- Cell closest to the source of the morphogen still assumes its fate
What happens in a mosaic organism in which one cell lacks a receptor for if the molecule is actually a true morphogen?
Bucket brigade?
True morphogen:
Cell will assume there is no morphogen and will assume default fates - disrupting patterning
Bucket brigade:
No affect - as long as the cell closest to the source of the molecule still has receptors present
What is passive diffusion?
The movement of molecules from a high to a low concentration
What is restricted diffusion?
- Bind to heparan sulphate proteoglycan molecules in the extracellular matrix
- High concentrations of receptor
- Prevents the molecules from diffusing away and generates a STEEP gradient
As well as restricted diffusion, what else generates a steep gradient?
How?
Rapid degeneration of the signal in the extracellular space
Prevents saturation of the environment and loss of the gradient
How does heparan sulphate proteoglycan act as a co-receptor?
Doesn’t actually bind the signal but keeps the signal around for longe enough for something else to receive it
Where do HSPGs bind?
Why is this beneficial?
To HSPG binding site on the ligand
Beneficial- keeps the receptor binding site on the ligand free
Where are HSPGs found?
In the extracellular matrix
In what 2 ways do HSPGs regulate morphogen diffusion?
1) Trapping/slowing diffusion
2) Facilitating diffusion
What is planar transcytosis?
What does this play a role in?
Transportation of molecules across the inside of the cell:
- Endocytosed at one end - into endocytic vesicle
- Moved across the cell
- Exocytosed at the other
- Taken up by the next cell in the same way
Plays a role in establishing SOME morphogen gradients
What is the evidence that transcytosis occurs in dpp (TGFbeta) signalling?
- Antibody staining shows Dpp is found in vesicles
AND - Mutations that block vesicle formation cause Dpp to act in a JUXATCRINE manner
How does timing play an important role in establishing morphogen gradients?
- Over time, the amount of molecules in any one space increases
- Causing MORE receptors to become occupied
- As the gradient is established - gene expression of the cells changes
- As the concentration of the morphogen increases
Why must there be a mechanism to block premature specification?
What does the cell do instead?
If not present - the cell would constantly be changing fate
The cell instead waits for a steady state of receptor activation to be achieved
What do the cells with specific fates have?
A specific transcription profile
Does the cell closest to the morphogen gradient have characteristics of all the other fates? (as it exceeds all the thresholds)
No
How is the morphogen gradient converted to a specific transcription profile within the cell?
Transcriptional read-out model:
- Receptor activation causes a TF to enter the nucleus and direct transcription (this is the SAME TF in every cell)
- BUT, higher concentration of morphogen results in a higher concentration of a activated transcription factor (TF) into the nucleus
What is bicoid?
How?
A morphogen AND a transcription factor
- Bicoid is present in drosophila, which has a syncytial blastoderm
- Bicoid protein diffuses throughout the cytoplasm and accumulates in the nuclei of the syncytial blastoderm
- Generating a concentration gradient
Where is bicoid mRNA localised?
At the anterior of the egg
How is the transcription factor concentration (from the transcriptional read-out model) interpreted at the DNA level?
Through an equilibrium between the on rate and off rate:
- Higher one rate with lower off rate - higher occupancy of TF
- Genes that specify the fate have a higher affinity for the TF than the genes that specify the fate of the other cells
- Both sets of genes see the same level of TF, but only the HIGH AFFINITY enhancers bind enough TF to activate gene expression
What happens at the level of gene transcription in the cell closest to the morphogen?
- Enough TF to activate bind to BOTH high and low affinity sites
- Genes that cause specify the fate of OTHER cells are now expressed
- Genes that specify the fate of cell 1 must BLOCK the expression of the other genes that specify the fate of other cells
How do the genes that specify the fate of cell 1 repress the genes that specify the fates of other cells?
What is this called?
Cell 1 fate genes encode a REPRESSOR of the genes that specify the fate of cell 2:
- This repressor binds to DNA upstream of cell 2 gene
- Blocks transcription of the gene
- Overrides the activation of the TF
This is called ‘cross-talk’
How are strict thresholds achieved when the gradient isn’t steep?
Positive feedback:
- Gene that specifies the fate of the cell is a DNA binding protein
- Binds to its own promoter
- More of the gene transcribed and translated - more repression of the genes that specify the fates of the cell next door
