Lecture 4 - morphogens Flashcards
(32 cards)
What are morphogens?
A soluble secreted molecule (can diffuse) that acts at a distance to specify the fates of cells
How can a morphogen specify more than one cell type?
by forming a concentration gradient
How can many cells be developed into different fates?
Through one instructive molecule. Concentration of ligand diffuses out and reduces in concentration - highest concentration by point source
What is the SOURCE?
where highest morphogen is found
What is the SINK?
where the morphogen has diffused to (lowest point)
Are a all molecules involved with patterning a morphogen?
No
Is a morphogen a permissive signal?
No, it is an instructive signal
How can test between an instructive and permissive signal?
- Provide a second source of the signal (e.g. at other end)
- Provide a signal at a uniform concentration
How can providing a second source of the signal (e.g. at the other end) establish whether a signal is instructive or permissive?
- The implantation of a second source - ECTOPIC SIGNALLING, will lead to mirroring of the pattern if it is instructive.
- If the signal is not instructive, then this means that there will be no change, as there are other signals that contribute - meaning it is permissive
How can providing a signal at a uniform concentration establish whether a signal is instructive or permissive?
- if the signal is instructive, all cells will adopt the same fate.
- if the signal is permissive, there will be no effect
Can a morphogen act indirectly?
No, a morphogen must act directly at a distance.
This means the same molecule must be present in low concentrations on cells further away to induce patterning, for it to be a morphogen.
If it induces another molecule which changes patterning - often referred to as the ‘bucket brigade’, then it is not a morphogen.
How can we distinguish whether a molecule acts directly, or whether it induces the next signal (bucket brigade)?
- use genetic engineering to make the proposed morphogen a juxtacrine, through the addition of a transmembrane domain.
- make a genetic mosaic that lacks the receptor for the red signal in one of the cells
How does making the morphogen a juxtacrine, through the addition of a transmembrane domain, establish between a directly acting molecule and one that induces the next molecule (bucket brigade)?
If the signal acts directly, there will not be any patterning off cells past the cell directly touching the cell producing the signal.
If the signal works through induction of the next molecule, then it would not be affected through the implementation of a transmembrane domain.
How does creating a genetic mosaic that lacks the receptor for the signal establish the difference between a signal acting directly, and a signal that induces the next molecule?
If the signal functions by acting directly, the cell which lacks a receptor will not be patterned.
If the signal functions by induction of the next molecule, then there will be no change
Is passive diffusion what establishes a morphogen gradient?
Passive diffusion would only generate a shallow gradient, so there must be other factors which establish a steep gradient
What factors establish a steep gradient?
While passive diffusion occurs, the main factors are:
- binding to molecules in the ECM (extracellular matrix) - e.g. heparan sulphate proteoglycans
- High concentrations of receptor
- Rapid degradation of signal
How do heparan sulphate proteoglycans (HSPGs) regulate morphogen activity?
HSPGs - aka. co-receptors regulate morphogens through:
- sequestration or slowing diffusion e.g. BMP ligands
- facilitating diffusion - e.g. Hedgehog ligands
What are examples of molecules that slow diffusion?
BMP ligands
What are examples of molecules that facilitate diffusion?
Hedgehog ligands
What does planar transcytosis do?
Plays a role in establishing some morphogen gradients.
This describe when 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 the cells in a tissue.
What is evidence for transcytosis?
In Dpp (TGF-beta) signalling: antibody staining shows that Dpp is found in vesicles AND mutations that block vesicle formation cause Dpp to act in a juxtracrine manner.
How is timing involved?
Timing plays an important role in establishing morphogen gradients.
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 - but the molecular mechanism for this ‘checkpoint’ is poorly understood.
How do cells read/interpret a gradient to make a cell fate decision?
There isn’t a mixture of cell type so there must be strict thresholds.
Describe the transcriptional read-out model
Higher concentrations of morphogen often results in a higher concentration of an activated transcription factor (TF).
In this model, receptor activation causes TF to enter the nucleus and direct transcription. It is the same TF in every cell - keep in mind these cells are initially identical. Different levels of TF in the nucleus lead to different cell fate decisions.
This is due to TF binding to enhancers with different affinities. This will lead to expression of different genes, and in-turn different cell fates.
