Lecture 4- Morphogens Flashcards
What is a morphogen?
A soluble secrete molecule that acts at a distance to specify the fates of cells. A morphogen may specify more than one type by forming a concentration gradient
What model do morphogens follow?
- Source: cell initiates and maintains production of morphogen
- Signal concentration gradient: morphogen diffuses away from the source down a concentration gradient
- Sink: cells furthest from the source who receive the morphogen at the lowest concentration
How is a cells fate determined by a morphogen threshold?
- Cells fate is determined by a threshold
* If the morphogen reaches the cell above a certain threshold, a certain fate is specified
What happened when biologist cloned ligands they thought might act as a morphogen?
- A gene mutated resulted in the loss of patterning
* But not all molecules involved with patterning are morphogens
Explain the pattern of cell fates with normal amount of morphogen
- As the amount of morphogen decreases, a new fate is specified
- The closer you are to the source/the higher the morphogen concentration, the more rapidly the cell fates will change with movement away from the source
- The further from the source/the lower the morphogen concentration, the cell fates will change less rapidly with movement away from the source
- The information is more dense by the source where the gradient is steeper
Explain the pattern of cell fates with increased levels of morphogens
- The diffusion away from the source results in all the same cell fate as they are all above the highest threshold
- So will gain more cells where their fate is dependant on higher levels of morphogen
- A similar pattern is then followed as under ‘normal’ morphogen levels
Explain the pattern of cell fates with reduced levels of morphogen
- Even at the highest level of reduced morphogen won’t be enough to specify certain cell fates
- Cell fates specified at higher concentrations/thresholds will be lost
- But there will be an increase in cells specified at low concentrations/thresholds
Explain the pattern of cell fates with a long ranged morphogen
- Instead of decaying quickly, the morphogen diffuses further (e.g more stable protein)
- The leads to a flatter concentration gradient
- Less cell fates specified overall as the gradient can only specify certain cell fates within a smaller range
Explain the pattern of cell fates where the morphogen is expressed ectopically
- Curved gradient
* Morphogen gradient moves away from the source but increases again as it moves closer to the ectopic source
Explain the pattern of cell fates where the morphogen is mutated?
- No morphogen so only cells which are specified by lack of morphogen will be observed
- No information so no pattern is developed
Explain the cell fate pattern where the morphogen is expressed uniformly
- Morphogen gradient does not change with a change in distance
- Cells will all have the same fate as all exposed to the same concentration of morphogen
- Information is the same so no cell fate pattern is observed
What 2 things must a morphogen do?
- Induce different outputs at different concentrations
2. Act directly at a distance
Explain how instructive signals are morphogens
The high concentration of morphogen induces a different output to lower concentration
Explain how permissive signals are not morphogens
A signal permits cells to respond to another source of information perhaps from a previously inherited cell fate determinants
Explain how providing a second source of the signal tests the hypothesis of morphogens as instructive and not permissive signals
- Using an ectopic morphogen
- Morphogen/instructive signal would result in mirror image of patterning
- Permissive signal would have no affect and show normal patterning of signal
Explain how providing a signal at a uniform concentration tests the hypothesis that morphogens are instructive signals and not permissive
- Morphogen/instructive signal would result in cells at the same fate
- Permissive signal would show no effect as the effect is pre-determined
Explain the bucket brigade morphogen theory compared to how morphogens actually act
Morphogen: can effect cells at a distance away from the source
Bucket brigade: cells provide information,ation to the adjacent cell which causes the adjacent cell to makes its own signal and provide information to the neighbouring cell to change its fate
Both theories suggest the first cell is responsible for patterning the field of cells. However, morphogen specific the fate of all cells, in the bucket brigade, each signal induces another
Explain how using genetic engineering to make a morphogen a juxtacrine tests the hypothesis that morphogens acts at a distance and the bucket brigade is incorrect
- Restrict the morphogen movement so signal can only be received by adjacent cells
- Other cells would all assume the fate that would be observed if no morphogen was present
- Bucket brigade would not be affect and cells could still provide information to adjacent cells
Explain how making a genetic mosaic that lacks the receptor for the first signal in one cell tests the hypothesis that morphogens acts at a distance and the bucket brigade is incorrect
- Make cell that lacks receptor for the morphogen released by the signalling cell
- The cell that lacks the receptor doesn’t specify to its normal fate as can’t ‘see’ the receptor
- The gradient still forms and doesn’t impact other cells so they still specify as normal
- Bucket brigade theory wouldn’t be affected by this
Is passive diffusion what establishes a morphogen gradient?
• Passive diffusion would only generate a shallow gradient and be inefficient
How are steeper morphogen gradients established?
- Binding to a molecules in the ECM (e.g HSPGs) and high concentrations of receptor can generate a steep gradient called restricted diffusion
- Rapid degradation of the signal in the extracellular space may also generate a steep gradient
What are HSPGs and what is their involvement in signalling?
- HSPGs are found in the ECM and bind to ligands
* Regulate diffusion by sequestration or slowing and facilitating diffusion
How does planar transcytosis help establish morphogen gradients?
- A pit forms in the cell membrane and engulfs the morphogen in a vesicle
- Repeated cycles of endocytosis and re-secretion through the cells allows certain morphogens to travel through the cell in a tissue and restricts its movement
What evidence is there for transcytosis in Dpp (TGFbeta) signalling
Antibody staining shows that Dpp is found in vesicles and mutations that block vesicle formation cause Dpp to act in a juxtracrine manner
What factors help establish a morphogen gradient?
- HSPGs
- Planar transcytosis
- Timing
- Rapid degradation of the signal in the extracellular space
- High concentrations of receptors
How does timing help establish morphogen gradients?
- The cells need to know when they should be reading the concentration of morphogen
- As the gradient is established, gene expression is changing with time
- There must be a mechanism to block premature specification. The cell probably waits for the steady state of receptor activation to be achieved- check point mechanism is poorly understood
What is the steady state of receptor activation?
The rate of degradation of secreted ligand is balanced by the rate of which new ligand is being produced
What model is used to describe how cells read or interpret a grained to make a cell fate decision?
Transcriptional read-out model
Explain the transcriptional read-out model
o High concentration of morphogen often results in higher concentration of an activated transcription factor
o Receptor activation causes TF to enter the nucleus and direct transcription
o It is the same TF in every cell (i.e same pathway/receptor/ligand)- these cells are initially identical
o Can convert the activity of an extracellular ligand to the activity of a nuclear localised TF
Explain how the transcriptional read-out model can be seen experimentally with Bicoid
- Bicoid is a morphogen and a TF
- Bicoid mRNA is localised at the anterior of the egg and is translated into a protein during early embryogenesis
- Bicoid protein then diffuses through the cytoplasm and accumulates in nuclei of the syncytial blastoderm generating a concentration gradient
How are strict thresholds achieves when the gradient is not steep?
Positive feedback
How is TF concentration interpreted at the DNA level?
o Enhancers that regulate one cell fate genes have lower affinity for the TF than those of a different cell fates
o At medium TF concentrations: even though both sets of genes see the same levels of TFs, only the high affinity enhancers can bind enough TF to activate gene expression of the cell with higher affinity
o Result: cell genes with higher affinity are activated
At high TF concentrations: both cell genes are activated as there is enough TF for binding in both the high affinity enhancer and low affinity enhancers
How does the affinity affect the TF + DNA ⇌ TF:DNA reaction?
o TF + DNA ⇌ TF:DNA (TF bound to DNA)
o Normal affinity binding: reaction is at equilibrium
o High affinity DNA binding: forward reaction is stronger, reverse reaction weaker. More TF:DNA (forward reaction favoured)
o Low affinity DNA binding: reverse reaction is stronger than forward reaction (reverse reaction favoured). Less TF:DNA, more TF + DNA.
What is cross talk regulation?
High morphogens switch on both cell genes, but one cell that encodes a repressor/switches off the genes of another cell
This is why cells don’t all have the same characteristics
