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BMS 249 - Developmental Neurobiology > Axon Guidance III > Flashcards

Axon Guidance III Flashcards

1
Q

How are complex axon pathways formed?

A

In several stages

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2
Q

How is the growth cone navigation to the final target broken into stages?

A

By intermediate targets/choice points

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3
Q

How are guidance cues used to guide axons to their targets?

A

In combinations

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4
Q

What must axons do at choice points and why?

A

They must ‘reprogram’ - alter their response to differnet cues

So that thy are no longer attracted to that choice point, but to the next intermediate target (choice point) on their journey to their final target

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5
Q

What is the same and different about the pathways of commissural axons in the hindbrain and in the spinal cord?

A

Both are guided to and cross the floor plate under the influence of netrin (released from the floor plate)

But, once they cross the midline:

  • Spinal nerves make a right angled turn, towards the brain
  • Hindbrain nerves continue past the floor plate (don’t make a turn)
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6
Q

What happens to commissural axons once they have crossed the floorplate?

How is this experimentally shown?

A
  • They lose their responsiveness to netrins and are no longer attracted to the floor plate
  • They become sensitive to something in the floorplate which is inhibitory!!

If the axon is exposed to ectopic floor plate (before it has crossed) it is attracted to it

If the axon is exposed to ectopic floor plate (after it has crossed) it does not extend towards it and it stalled

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7
Q

What can fluorescent lipophillic dyes do?

A

Can highlight cell membranes (can track axons and their lineage)

Can come in many different colours

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8
Q

What are the inhibitory factors in the floor plate?

A

Semaphorins and slits

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9
Q

Where are semaphorins and slits expressed (as well as in the floor plate)?

What does this form?

A

Motor pools of the ventral spinal cord

Forms a channel where commissural axons grow through

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10
Q

What happens to the sensitivities of commissural axons as they travel to and cross the floor plate?

A

Initially sensitive to netrins and not to slit or semaphorins

Then, after crossing the midline, is sensitve to slits and semaphorins and not netrins

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11
Q

What is common and not common about the nervous system in vertebrates and invertebrates?

A

Common:
Midline

Not common:
Vetebrates - dorsal nerve cord
Invertebrate - ventral nerve cord

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12
Q

What pathways can axons take in the body?

A
  • Extend longitudinally, without crossing the midline
  • Start to cross the midline, but the extend longitudinally
  • Cross the midline and join longitudinal paths by axons which have not crossed the midline
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13
Q

What occurs in a ROUNDABOUT mutant?

A

There are very few longitudinal tracts

Commissural axons cross and recross multiple times

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14
Q

What is a commissure?

A
  • A bundle of commissural axons that extend across the midline (ventral nerve cord) to connect the lateral nerve pathways on either side of the body - forming a ladder like structure
  • They are regularly spaced
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15
Q

What occurs in a COMMISSURELESS mutant?

A
  • Commissural axons fail to cross the midline at all

- They remain in the longitudinal fasicles on the same side as the cell body

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16
Q

What does Robo encode?

A

A receptor for the inhibitory protein slit

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17
Q

Where is Robo expressed in high concentrations?

A

In Axons which DON’T cross the midline

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18
Q

What happens to the level of Robo when they cross the midline?

A

Robo levels increase (more receptors - more sensitive to slit)

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19
Q

Why do ROBO mutants cross the midline many times?

A

No Robo receptors to detect Slit (inhibitory) in the floor plate

Commisures are continuously attracted to and crossing the midline

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20
Q

Where is COMM expressed?

A

In neurons which normally DO cross the midline

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21
Q

What happens to COMM expression when neurons cross the midline?

A

It is switched off

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22
Q

How is it shown that comm controls robo levels?

A

In comm mutants (no comm):
Robo protein is upregulated in neurons which normally cross the midline - these axons extend longitudinally (don’t cross the midline)

When comm is upregulated in ALL neurons:
Robo protein is lost everywhere, robo mutant
All axons cross the midline multiple times

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23
Q

How does COMM control ROBO?

A
  • COMM encodes a trafficking protein which prevents ROBO from reaching the cell surface of the growth cone
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24
Q

What happens to COMM when the axons cross the midline?

What does this cause?

A
  • COMM decreases
  • The amount of ROBO receptors on the growth cone surface increases
  • Growth cone becomes sensitive to Slit
  • Growth cone no longer can respond to netrins and is repelled from the midline, to prevent recrossing
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25
Q

What is the vertebrate homologue of comm?

A

There isn’t one

26
Q

Where is COMM and ROBO found (which organism)?

A

Invertebrates (drosophila)

27
Q

What is the vertebrate homologue of ROBO and when is it expressed?

A

Robo1

Expressed on commissural axons, both before and after they cross the midline

28
Q

What is Rig1 and what does it do?

A

A robo-like protein which is ONLY expressed in the pre-crossing fibres and appears to block Robo1 signalling until the midline is crossed

29
Q

In vertebrates, what happens in Rig1 mutants?

A

Commissural axons are attracted to the floorplate but cannot cross

Axons are prematurely sensitive to slits

30
Q

How do axons stay on the axons scaffold and get off the axon scaffold to reach the target?

A

By controlling FACICULATION

31
Q

What are the interactions between bundled axons in a fasciculation?

A

Homophillic binding by cell adhesion molecules CAMs

32
Q

What is an example of a CAM in invertebrates?

A

Fas II

33
Q

Where is Fas II found and what does it do?

A
  • Sits on top of the membrane

- Binds homophillically to link 2 cell surfaces together

34
Q

If Fas II is expressed in cells which do not normally adhere, what can happen?

A

These cells can aggregate together

35
Q

In the ventral nerve cord, where is Fas II found?

A

In the longitudinal tracts

36
Q

What happens in Fas II mutants?

A

Many defasciculated axons in the longitudinal tracts

37
Q

What happens when Fas II is over expressed?

A
  • Novel fasciculations - tracts which are normally separated are joined together
  • By-Pass phenotype
38
Q

How are muscles divided in the body?

A

Into muscle blocks

39
Q

As well as fasiculation, what does Fas II control?

A

Defasciculation

40
Q

What is a “By-Pass phenotype”?

A

Caused by over-expression of Fas II:

- Axons continue onto the periphery and fail to get off the main tract to innervate the muscle

41
Q

What can Fas II be regulated by and what does this do?

A

BEAT - interferes with CAM-mediated adhesion (homophilic adhesion)

Allowing axons to leave the longitudinal pathway and go into the periphery

42
Q

What are 2 types of target of extending axons?

A

1) Cellular/ Discrete targets (specific cell)

2) Multicellular/ Topographic maps (Finding a specific area)

43
Q

What proves that some axons find discrete targets?

A
  • Ablation of specific target muscles in the drosophila/grasshopper leads to the failure of relevant motor axons to leave the motor trunk at the appropriate branch point
44
Q

What are the ‘address label’ signals that are expressed on cells in the invertebrate?

A

1) Netrins

2) Fas III

45
Q

Where happens when netrin is lost from the muscles?

A
  • Like ablating the muscles - axons wander and do not make synapses
46
Q

What is netrin?

A

A diffusible chemoattractant which is present on specific muscle cells of the body

47
Q

What happens if netrin is expressed ectopically?

A

Leads to axons innervating the wrong muscles

48
Q

How are ‘address labels’ made?

A

By multiple cues interacting together

49
Q

What is Fas III?

A

A contract attractant

Homophillic adhesion molecule

50
Q

Where is Fas III expressed?

A

On BOTH:

  • Specific muscles
  • And motor axons that innervate them
51
Q

What does ectopic expression of Fas III lead to?

A

Fas III expressing axons to innervate these new targets

52
Q

What were Sperry’s proposed probabilities on how topology of the retina maintained in the tectum?

A

1) Each axon has a unique label complimentary to a unique label on the target
2) Co-ordinate system, encoded by GRADIENTS of signalling molecules and GRADIENTS of receptors expressed in the retinal ganglion cells

53
Q

What did the stipe assay show in the tectum and how?

A
  • Cells in the posterior tectum make a non-permissive factor that repels TEMPORAL retinal axons

Nasal neurons grew on BOTH anterior and posterior stripes of membrane

Temporal neurons only grew on ANTERIOR stripes of membrane (posterior membrane causes the growth cones to collapse)

54
Q

What is the inhibitory factor in the posterior tectum and how is this expressed?

A

2 ephrins (A2 and A5)

Expressed in a gradient (High at the posterior)

55
Q

What is the receptor to the inhibitory factor in the posterior tectum and how is this expressed?

A

EphA

Expressed in a counter gradient to the ephrin ligand (High at the temporal retina - posterior of the eye)

56
Q

Why was Sperry’s first proposed probability likely to be incorrect?

A
  • Would have to have MANY gene to produce many individual target labels
57
Q

What happens in the stipe assay when the stripes were heated?

A
  • Activity of repellants abolished in posterior stripes (axons could grow)
  • BUT nothing happened differently to the anterior stripes - shows they are not producing a substance
58
Q

How does the graded expression of ephs and ephins the the retina/tectum cause the graded position of the neurons?

A
  • More receptors for inhibitory signal
  • More sensitive to the inhibitory signal (especially at high concentrations)
  • Repelled further away
59
Q

In mice with Ephrin A2 and A5 knockout, what happens?

What does this mean?

A

Temporal neurons can project their axons onto the posterior tectum, disorganising the topographic map

Non-permissive, repellant factors can be used instructively to form topographic maps

60
Q

How is nerve navigation in vertebrate and in invertebrate the same?

A
  • Both have diffusible attractants and repellants

- Axons crossing form commissures, then turn to join the longitudinal tracts

61
Q

In regards to comm and robo, which 2 mutations have the same phenotype?

A

Upregulation of Comm

Inhibition of Robo

62
Q

Where is Fas II found?

A

In invertebrates

BMS 249 - Developmental Neurobiology (16 decks)

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