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BMS 242 - Principles of Developmental Biology > Morphogens > Flashcards

Morphogens Flashcards

1
Q

What is a morphogen?

A

A SOLUBLE, SECRETED molecule that acts at a DISTANCE to specify the FATES of cells

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2
Q

How does a morphogen specify more than one type of cell?

A

Through a concentration gradient

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3
Q

Describe morphogen gradients

A
  • 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
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4
Q

What happens if there is an ectopic source of morphogen?

A

There is a mirror image of pattern formation

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5
Q

Are all molecules that are involved with patterning morphogens?

A

No

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6
Q

What are the possible function of a diffusible ligand?

What type of signal is this?

A

Tells the cell to assume a fate they already know, by permitting the cell to respond to another source of information

‘Permissive signal’

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7
Q

Are permissive signals morphogens?

Why?

A

No

Permissive signals do not provide information - they just act as a ‘switch’

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8
Q

What 2 things MUST a molecule do to be a morphogen?

A

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

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9
Q

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’

A

Using 2 tests:

1) Provide a second source of the morphogen
2) Provide the signal at uniform concentrations

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10
Q

Describe what happens if the is a second concentration of a true morphogen?

A
  • Mirror image of cell fates

- Loss of fates that respond to LOW concentrations of morphogen

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11
Q

Describe what happens if the is a second concentration of a permissive signal?

A

Cells will still assume their original fate

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12
Q

How was Shh discovered in the wing bud?

What does this show?

A

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

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13
Q

What happens if you provide an instructive signal at a uniform concentration?

Permissive signal?

A

Instructive signal - All cells will adopt the SAME fate (all see the same concentration of morphogen)

Permissive signal - no difference

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14
Q

What is another name for a morphogen?

A

Instructive signal

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15
Q

What is the ‘bucket brigade’?

What molecule is this method not used for?

A

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

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16
Q

How do morphogens act at a distance?

A

Directly:

  • Released from one cell
  • Patterns following cells in a concentration-dependant manner
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17
Q

How do we distinguish if a molecule is acting over distance directly or indirectly (bucket brigade)?

A

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

18
Q

What happens if the signalling molecule is juxtacrine, if the molecule was a morphogen?

A
  • 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
19
Q

What happens if the signalling molecule is juxtacrine, if the molecule signalling using a bucket brigade system?

A
  • Patterning not affected

- Cell closest to the source of the morphogen still assumes its fate

20
Q

What happens in a mosaic organism in which one cell lacks a receptor for if the molecule is actually a true morphogen?

Bucket brigade?

A

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

21
Q

What is passive diffusion?

A

The movement of molecules from a high to a low concentration

22
Q

What is restricted diffusion?

A
  • 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
23
Q

As well as restricted diffusion, what else generates a steep gradient?

How?

A

Rapid degeneration of the signal in the extracellular space

Prevents saturation of the environment and loss of the gradient

24
Q

How does heparan sulphate proteoglycan act as a co-receptor?

A

Doesn’t actually bind the signal but keeps the signal around for longe enough for something else to receive it

25
Q

Where do HSPGs bind?

Why is this beneficial?

A

To HSPG binding site on the ligand

Beneficial- keeps the receptor binding site on the ligand free

26
Q

Where are HSPGs found?

A

In the extracellular matrix

27
Q

In what 2 ways do HSPGs regulate morphogen diffusion?

A

1) Trapping/slowing diffusion

2) Facilitating diffusion

28
Q

What is planar transcytosis?

What does this play a role in?

A

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

29
Q

What is the evidence that transcytosis occurs in dpp (TGFbeta) signalling?

A
  • Antibody staining shows Dpp is found in vesicles
    AND
  • Mutations that block vesicle formation cause Dpp to act in a JUXATCRINE manner
30
Q

How does timing play an important role in establishing morphogen gradients?

A
  • 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
31
Q

Why must there be a mechanism to block premature specification?

What does the cell do instead?

A

If not present - the cell would constantly be changing fate

The cell instead waits for a steady state of receptor activation to be achieved

32
Q

What do the cells with specific fates have?

A

A specific transcription profile

33
Q

Does the cell closest to the morphogen gradient have characteristics of all the other fates? (as it exceeds all the thresholds)

A

No

34
Q

How is the morphogen gradient converted to a specific transcription profile within the cell?

A

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
35
Q

What is bicoid?

How?

A

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
36
Q

Where is bicoid mRNA localised?

A

At the anterior of the egg

37
Q

How is the transcription factor concentration (from the transcriptional read-out model) interpreted at the DNA level?

A

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
38
Q

What happens at the level of gene transcription in the cell closest to the morphogen?

A
  • 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
39
Q

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?

A

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’

40
Q

How are strict thresholds achieved when the gradient isn’t steep?

A

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

BMS 242 - Principles of Developmental Biology (15 decks)

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