Lecture 4: morphogens Flashcards
What is a morphogen?
A soluble secreted molecule that acts at a distance to specify cell fates depending on the concentration that the cell receives. To be a morphogen, a molecule must form a concentration gradient, inducing different outputs at different concentrations; it must be instructive and not permissive; and must act directly at a distance.
How do you prove a molecule is a morphogen?
A morphogen is instructive. If it was permissive it would simply permit cells to respond to another source of information. This can be tested by providing an ectopic source of the molecule at the opposite end of the original source. A morphogen would produce a mirror image phenotype whereas for a permissive signal there would not be any change.
A morphogen acts directly at a distance. It does not signal as a bucket brigade i.e., one signal causes the release of another. This can be tested in two ways:
- if the receptor for the molecule is not known, it can be genetically engineered to add a transmembrane domain. This would mean the molecule now signals in a juxtracrine manner and therefore if it was a morphogen, it would get stuck in the surrounding cells. A bucket brigade would not be effected because the molecule simply causes release of a different molecule from that cell.
- if the receptor for the molecule is known, a genetic mosaic of cells that lack the receptor can be made. If the molecule was a morphogen, there would be a gap in the morphology whereas if it signalled in a bucket brigade system, morphology would not be effected (unless the receptor was knocked out of surrounding cells).
Explain how a gradient is established.
Restricted diffusion
Restriction to one location ensures the molecule doesn’t drift away in all directions like in passive diffusion. There is a high concentration of morphogen receptors on target cells and rapid degradation of the morphogen.
Heparan sulphate proteoglycans (HSPGs)
Found in the EC matric. Bind many morphogens at low affinity to regulate diffusion.
Planar transcytosis
A pit forms in the cell membrane and engulfs the morphogen into a vesicle. Repeated cycles of endocytosis and re-secretion restricts morphogens to a specific tissue.
Give evidence for planar transcytosis.
Decapentaplegic (dpp) in Drosophila or TGFbeta in mammals are morphogens that use transcytosis. Antibody staining shows that dpp is found in vesicles. Mutations that block vesicle formation cause dpp to act in a juxtracrine manner.
Why is bicoid unique?
Bicoid is a morphogen and a TF. In early drosophila development, the nuclei divide but the cell does not i.e., it is a multi-nucleated single cell. A syncytium allows morphogens to diffuse across the embryo that has no boundaries. Bicoid determines the anterior axis. mRNA for Bicoid is deposited at the anterior in high concentrations by the mother and when translated it diffuses down the embryo.
Explain the transcriptional read-out model.
Lets say that we have a theoretical morphogen which causes a TF to enter the nucleus, remembering that cells are initially identical i.e., it is the same TF in each. Cells receive different concentrations of the morphogen. Higher concentration of morphogen = more TF activated = more TF enter the nucleus.
The cell further away from the source of the morphogen has fewer TF activated and therefore only high affinity enhancers can be bound. This enhancer switches on the gene that gives this cell its identity.
The cell closer to the source of the morphogen has more TF activated and therefore both high and low affinity enhancers can be bound. Since the differing factor between the two cells is activation of the low affinity enhancer, it must encode a repressor for the high affinity enhancer, inhibiting the fate of the cell further away.
The cell further away has a positive feedback mechanism i.e., it encodes a TF which activates its own expression (among other things).
