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NEUR 310 > Axon outgrowth and Guidance > Flashcards

Axon outgrowth and Guidance Flashcards

1
Q

What first left neuroscientists the impression that growth cones moved in an ordered and directed manner?

A

Golgi’s drawings.

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2
Q

True or false: most retinal axons do not respond to more than one axon guidance cue to reach to optic tectum.

A

False, there are at least 10 steps in this process.

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3
Q

Complete the information about the retinotectal pathway with the word bank.

dorsal
temporal
caudal
dorsal
rostral

Axons leaving the _____ part of the retina end in the ventral part of the optic tectum.

Axons leaving the ventral part of the retina end in the _____ part of the optic tectum.

Axons leaving the nasal of the retina end in the _____ part of the optic tectum.

Axons leaving the _____ part of the retina end in the _____ part of the optic tectum

A

Axons leaving the dorsal part of the retina end in the ventral part of the optic tectum.

Axons leaving the ventral part of the retina end in the dorsal part of the optic tectum.

Axons leaving the nasal part of the retina end in the caudal part of the optic tectum.

Axons leaving the temporal part of the retina end in the rostral part of the optic tectum.

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3
Q

What does the random outgrowth theory has to say about axon pathfinding?

A

Axons would not follow cues to reach a specific target. Instead, there would be random axonal outgrowth in all directions, and only the relevant connections would be preserved by experience-dependent mechanisms.

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4
Q

What is the chemoaffinity hypothesis postulating?

A

Neurons express different combinations of receptors and molecular markers, which allows them to follow specific cues and make specific connections with biochemically relevant targets.

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5
Q

What famous experiment debunked the random outgrowth theory of neuronal development? Describe the experiment.

A

The Sperry experiment. The projections from the retina to the optic tectum were severed for one eye of a xenopus. The eye was then flipped 180 degrees. Projections from the flipped eye regrown to the same regions of the optic tectum, which supports the chemoaffinity hypothesis.

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6
Q

Provide an example of transplant experiment in the visual system that proves the chemoaffinity theory.

A

When transplanting patches of neuroepithelium from a donor tadpole to the optic tract of a host, the axons will always grow toward the region of the patch where they bind under natural conditions.

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7
Q

How can a growth cone be visualized?

A

By transfecting it with GFP-actin.

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8
Q

True or false: in a typical growth cone, extensions are microtubule-rich while the core is actin-rich.

A

False: the opposite.

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9
Q

What is driving the extension of the growth cone?

A

Actin polymerization.

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10
Q

What makes growth cones sensible to their environment?

A

Presence of receptors.

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11
Q

What are the two types of “extension” in a growth cone? What are they constituted from?

A

Lamellipodia (sheet-like) and filopodia (finger-like). Made from actin.

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12
Q

What is controlling growth cone steering?

A

Microtubule depolymerization and polymerization.

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13
Q

How is cytochalasin modulating actin polymerization?

A

Prevents actin polymerization by binding to + end of actin filament.

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14
Q

Since actin is treadmilling in the extensions (filopodia and lamellipodia) of the growth cone, actin-poor spaces are created when actin polymerization stretches the plasma membrane forward. How are those spaces solidified?

A

The microtubules from the central core advance to occupy such spaces.

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15
Q

What are the growth cone attractive and repulsive cues regulating specifically?

A

They are regulating the rates of actin polymerization and depolymerization.

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16
Q

Which component of the extracellular matrix mainly promotes axon outgrowth ability?

A

Laminin.

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17
Q

Propose a simple experiment to verify if a laboratory-made ECM promotes neurite outgrowth.

A

Culture neurons on a patterned substrate of the artificial ECM segmented in ECM-coated corridors and inert ECM-free patches. If the artificial ECM promotes neural outgrowth, growth cones should extend on the ECM-coated surface.

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18
Q

True of false: integrins bind to laminins.

A

True.

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19
Q

What is one characteristic that differs different types of integrin?

A

The laminin types to which they bind (determined by subunit composition).

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20
Q

Give one documented example of laminin guiding axons in vivo.

A

Laminin guiding retinal ganglion cells axons across the vitreal surface to the entrance of the optic nerve in the retina.

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21
Q

How can antibodies be used to isolate cell adhesion molecules (CAMs)?

A

If antibodies are added to dispersed cells in suspension that supposedly express CAMs and those cells cannot cluster when brought in proximity, it is likely to imply that antibodies successfully bound CAMs and prevented cells to stick to each other.

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22
Q

What type of calcium-dependent cell adhesion molecules is extensively found in the nervous system?

A

N-Cadherin.

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23
Q

True or false: some CAMS that belong to the immunoglobulin family are calcium dependent.

A

False: all molecules are calcium-independent.

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24
Q

Why do CAMs require intracellular domains?

A

To convey the extracellular signal intracellularly.

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25
Q

True of false: since ephrins are ligands, they do not have intracellular domains.

A

False: some of them do, like ephrin B.

26
Q

In the case of ephrins, what is forward signaling? What is reverse signaling?

A

Forward signaling is the activation of signaling pathways downstream of Ephs (receptors) upon ligand binding.

Reverse signaling is the activation of signaling pathways downstream of Ephrins (ligand) upon binding to their receptors.

27
Q

What type of ephrin signaling prevents RGCs from entering the posterior region of the optic tectum? Is it repulsive or attractive?

A

Ephrin A - Eph A: repulsive.

28
Q

Complete the information about the axon guidance ecology of RGCs with the word bank.

ventral x 2
nasal
retina
posterior
anterior
EphrinB
dorsal
temporal
optic tectum

The _____ gradient in the retina goes ([high] to [low]) from _____ to dorsal.

The EphA gradient in the _____ goes ([high] to [low]) from _____ to _____.

The EphrinA gradient in the _____ goes ([high] to [low]) from _____ to _____.

The EphB gradient in the optic tectum goes ([high] to [low]) from _____ to _____.

A

The EphB gradient in the retina goes ([high] to [low]) from ventral to dorsal.

The EphA gradient in the retina goes ([high] to [low]) from temporal to nasal.

The EphrinA gradient in the optic tectum goes ([high] to [low]) from posterior to anterior.

The EphrinB gradient in the optic tectum goes ([high] to [low]) from dorsal to ventral.

29
Q

Are all ephrin signals repulsive?

A

No, EphrinBs are attractive for example.

30
Q

Give one documented example of ephrins guiding axons in vivo.

A

EphrinA-EphA (repulsive signaling) and EphrinB-EphB (attractive signaling) mediate the targetting of RGCs in the optic tectum.

31
Q

What are the two types of membrane attachment that we find in ephrins? Ephrins A and B rely on which types?

A

Ephrins are either GPI-anchored to the plasma membrane or have cytoplasmic domains. Ephrin As are GPI-achord while Ephrin Bs have cytoplasmic domains.

32
Q

Ephrin Bs or As are capable of reverse signaling?

A

Ephrin Bs.

33
Q

Why are some forms of semaphorin very efficient to establish a gradient?

A

Some forms (Sem3) are produced by the cells and can therefore easily diffuse.

34
Q

What is the consequence of eliminating the intracellular domain of neuropilins (Nrp)? What does that indicate?

A

No change. A coreceptor is needed (plexin).

35
Q

True of false: some semaphorins can signal directly through plexins without having to bind to Nrp.

A

True.

36
Q

What component of semamorphin systems mediates intracellular signaling?

A

Plexin coreceptors.

37
Q

What signal guides RGCs along the optic nerve? Is it attractive or repulsive?

A

Sema5A, repulsive.

38
Q

What signal is in charge of preventing axons from wandering in the optic chiasm?

A

Slit signaling.

39
Q

Describe the trajectory of commissural axon neurons in the neural tube of the mouse?

A

Roof plate of neural tube (caudal) -> floor plate of neural tube -> crossing midline -> projecting rostrally along the floor plate.

40
Q

In commissural guidance, how is netrin involved?

A

Attractive signal of the floor plate.

41
Q

Is netrin GPI-anchored or does it have a cytoplasmic domain?

A

Neither, not bound to plasma membrane.

42
Q

What kind of response does netrin binding to DCC receptors provoke? Under which conditions is that reversed?

A

Attractive. Reversed when Unc5 is coexpressed.

43
Q

What is the receptor of slit?

A

Robo.

44
Q

Name two morphogens involved in the patterning of the nervous system.

A

Shh and Wnt.

45
Q

In what other process are involved morphogens?

A

Cellular migration.

46
Q

What intracellular family of proteins is of particular importance for controlling growth cone morphology by translating extracellular signals into intracellular events?

A

Rho GTPases.

47
Q

Rac, Cdc-42 and Rho all regulate different aspects of the growth cone morphology. For which aspect is each Rho protein GTPase responsible for?

A

Activation of Rac: promotes expension of lamellipodia.
Activation of Cdc-42: promotes expension of filopodia.
Activation of Rho: promotes actin depolymerization.

48
Q

What are doing GDP dissociation inhibitors?

A

Bind to the Rho GTPase protein in its GDP form and lock it into that state.

49
Q

What is ephexin?

A

A GEF for multiple Rho GTPases (promotes GTP form of the latter).

50
Q

What Rho GTPases is ephexin activating?

A

Mainly Rho and Cdc-42 (= collapse).

51
Q

The Slit-Robo pathway has which impact on the Rho GTPase system?

A

Promotion of GAP activity in the Cdc-42 GTPase cycle (= deactivation).

52
Q

List three broad factors that determine the response of neurons to axon guidance cues.

A
  1. Neuronal type.
  2. Environment.
  3. Developmental stage.
53
Q

Are membrane receptors associated with axon guidance cues usually directly interacting with Rho GTPase proteins? If not, what is the intermediate step?

A

No, they usually modulate the activity of GEFs, GAPs and GDIs, and those directly interact with Rho GTPases.

54
Q

Explain the paradigm-shifting experiment that was interested in understanding how neurons adapt their responses to axon guidance cues in an appropriate spatio-temporal manner during development.

A

Addition of a PKA inhibitor (major downstream effector of cAMP) to a petri dish containing Xenopus spinal neurons and netrin reversed the neurons’ valence to netrin.

55
Q

If you wanted to change the valence (not eliminate) of a neuron to a guidance cue, what would you target intracellularly?

A

The ratio of intracellular cAMP to cGMP.

56
Q

Netrin binding to a dimer of DCC and Unc-5 receptors leads to what kind of response for the growth cone?

A

Repulsion.

57
Q

What simple experiment could be done to show that protein synthesis is necessary for RGC responsiveness to chemotropic cues?

A

Add reagents impairing the protein synthesis machinery.

58
Q

True or false: components of translational machinery are only found in the nucleus of a neuron.

A

False: they are at least also found in the growth cone during development.

59
Q

What is one signal (guidance cue) that prevents commissural neurons from re-entering the floor plate once they’ve crossed it?

A

EphrinA-EphA.

60
Q

What could be a mechanism to regulate receptor expression in a spatio-temporal manner for neurons in outgrowth?

A

Environmentally-induced transcription of dormant mRNA in the growth cone.

61
Q

What extracellular post translational mechanism can modify the responsiveness of neurons to guidance cues?

A

Metalloproteases by clipping off receptors from the neuron’s surface.

62
Q

What is preventing commissural neurons from retrogradely crossing the midline once they’ve crossed it? Isn’t netrin (floor plate signaling) still attracting them towards the midline?

A

Once commissural neurons cross the midline, Robo receptors are upregulated and their intracellular domains interact with DCC’s domains to silence them. Therefore, there is no more competing cuing activity and neurons respond almost exclusively to Slit signaling.

NEUR 310 (12 decks)

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