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Fundamental Neuroscience > Axon growth and guidance > Flashcards

Axon growth and guidance Flashcards

1
Q

What does the growth cone do?

A
  • Explores extracellular environment by pushing forward and retracting
  • Guides axon extension of neuron beind hit as well
  • Determines direction of growth
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2
Q

Label this image. What is it?

Re-do this question please

A

C

B

A

Growth cone

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3
Q
A
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4
Q

What is the growth cone made of?

A

F-actin and microtublues

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5
Q

What does the filopedia of the growth cone do?

A

Rapidly extends and withdraws

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6
Q

What are the 3 main domains of the growth cone? (3 marks)

A
  1. Peripheral domain
  2. Transition zone/ domain
  3. Central domain
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7
Q

What does the peripheral domain of the growth cone contain?

A
  • Filopedia which is a linear bundle of F-actin
  • Lamellipodia - located inbetween filopedia andis a mesh like network of F-actin
  • Dynamic pioneer microtubules
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8
Q

What does the transition zone contain?

A

F-actin archs which are 90º to F-actin bundles

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9
Q

What does the central domain contain?

A

Dynamic microtubules that form bundles

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10
Q

What structure is located behind the central domain?

A

Axon shaft

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11
Q

What are the F-actin in filopedia and microtubules in the central domain associated with? What does this molecule do?

A

Motor proteins - these transport the vesicles and macromolecules up and down F-actin and microtubules

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12
Q

Which strucutre doe anterograde and retrograde mvoement go towards to in the growth cone skeleton?

A

Anterograde: towards filopodia tips

Retrograde: towards neuron soma

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13
Q

Label the image below of the growth cone cytoskeleton. Include:

  • Axon shaft
  • Filopodium
  • F-acrin arc
  • F-actin bundle
  • Stable microtubules
  • F-actin network
  • Dynamic pioneer microtublues
  • Lamellipodia like veil

And the 3 different domains

A
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14
Q

What structure are the f-actin filaments in filopodia aligned to form?

A

Double helix

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15
Q

Where is ATP G-actin and ADP G-actin added and removed in the f-actin filament?

A

At the positive end of the f-actin molecule, ATP G-actin is added and at the negative end ADP G-actin is removed

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16
Q

How is a steady retrograde flow induced in f-actin and what does this result in the cleavage of?

A

Myosin will bind to f-actin causing retrograde flow. This will cause continuous cleavage of ADP G-actin at the negative end of the f-actin filament, which will get recylced to ATP G-actin

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17
Q

What happens in the f-actin filament in the absence of adhesive contacts? What is the name of this process?

A

No net growth and elongation of f-actin filaments

Actin treadmilling

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18
Q

What interactions will slow retrograde f-actin flow and promote filopedia extension?

A

Cell/cell interactions by cell adhesion molecules (CAM).

e.g. Ca2+ independent CAMS (NCAM & L1)

Ca2+ dependent CAMS (cadherins)

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19
Q

How does an indirect interaction of integrin and f-actin occur?

A

ECM have to be bound to filopodia expressed integrin

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20
Q

What does the ‘molecular clutch’ do in the filopodia?

A

Either slows or prevents local retrograde flow of F-actin

and reduces the release of ADP G-actin at the minus end

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21
Q

What are microtubules made of? (3 marks)

A

Alpha and ß tubllin dimers in protfilaments. 13 protofilaments are arranged around a hollow core to form microtubules with a diameter of 24 nm

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22
Q

What is the difference in the positive and negative ends of the microtubules?

A
  • Positive end is fast growing and has ß tubulin
  • Negative end is slwo growing and has α tubulin
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23
Q

What is the process of dynamic instability and what happens? (5 marks)

A
  • Promotes microtubule growth
  • α/ ß tubulin GTP dimers added to the positive end in polymerisation (MT growth)
  • α/ ß tubulin GDP dimers are removed from the positive end in depolymerisation (MT catastrophe)
  • α/ ß tubulin GDP dimers are converted into GTP dimers for re-polymerisation (MT rescue)
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24
Q

How is polymerisation balanced and regulated in dynamic instabililty?

A

Microtubule association

25
Q

What needs to happen for the molecular clutch to form?

A

Growth cone needs to encounter an adhesive substrate

26
Q

What are the four stages of axon growth?

A
  1. Adhesive substrate encounter
  2. Protusion
  3. Engorgement
  4. Consolidation
27
Q

What happens in the first stage of axon growth?

A

Adhesive substrate encounter:

  • Formation of molecular clutch in filopodia
  • Slow local retrograde flow of F-actin
28
Q

What happens in the second stage of axon growth?

A

Protusion:

  • Due to no retrograde flow, F-actin polymerisation will drive filopdia and lamellipodia like veils forward
  • (Pioneer MTs guided to extend filopodia by the F-actin bundles, transition zone will also extend)
29
Q

What happens in the 3rd stage of axon growth?

A

Engorgement:

  • Dynamic central domain MTs guided into extended TZ by actin arcs (causes extended central domain)
30
Q

What happens in the 4th stage of axon growth?

A

Consolidation:

  • New axon shaft froms behind advancing CD due to MT stabilization
31
Q

What do actin nucleators do?

A

Form G-actin trimers prior to F-actn assembly

32
Q

What do F-actin capping proteins do?

A

Inhibit F-actin assembly at positive end of filaments and inhibit G-actin cleavage at negayive end

33
Q

What do F-actin polymerisation factors do?

A

Promote F-actin assembly and prevent capping proteins from binding to positive end of F-actin

34
Q

What do F-actin severing proteins do?

A

Remove ADP G-actin from positive end of F actin and cleave actin filaments

35
Q

What do monomer binding proteins do?

A

Transport ADP G-actin from minus end of F-actin back to hte plus end and convert ADP G-actin to ATP G-actin

36
Q

What do motor proteins do?

A

Mediate retrograde flow of F-actin and retrograde and anterograde transport of vesicles and macrmolecules

37
Q

What do F-actin bidning proteins do?

A

Bundling of F-actin filaments, F-actin cross-linking in mesh like networks and link F-actin to plasma membrane

38
Q

What are the proteins that regulate growth cone motility and growth cone turning? (6 marks)

A
  • F-actin capping proteins
  • F-actin polymerisation factors
  • F-actin severing proteins
  • Monomer binding proteins
  • Motor proteins
  • F-actin binding filaments
39
Q

What do plus end tracking proteins do?

A

Bind to the postiive ends of dynamic, polymerising MTs to stabilise newly added α/ ß tubulin dimers

e.g. APC, APC2, CLASP

40
Q

What do microtubule stabilising proteins do?

A

Bind to existing microtublues to stabilise their structure

e.g. MAP1B, TAU

41
Q

What do MT sestablisng proteins do?

A

Induce MT depolymerisation (catastrophe) counteracting microtubule polymerisation/ elongation

42
Q

What do MT severing proteins do?

A

Induce microtubule cleavage important in axon branching

43
Q

What do motor proteins do?

A

Retrograde (dynein) and anterograde (kinesin 5) transport of vesicles and macromolecules alonfg axon

44
Q

What are the four types of molecular guidance cues? (8 marks)

A
  1. Adhesive substrate bond cues - short range contact attraction
  2. Repellent substrate bond cues - short range - prevent axon veering off straight course
  3. Diffusible chemoattractive cues - long range - pull growth cone towards the right
  4. Diffusible chemorepulsive cues - long range - push growth cone from behind to the right
45
Q

What do adhesive substrate bound cues consist of?

A

Ca2+ independent cell adhesion molecules e.g. NCAM

Ca2+ dependent cell adhesion molecules e.g. cadherins

ECM proteins

All of the above promote axon growth

46
Q

What are netrins and what do they do?

A

Secreted diffusible proteins - long range

Netrin 1 and 2 are chemoattractive cues for growth cones and express DCC

Also act as chemorepellent cues by expressing UNC 5

47
Q

What are semaphorins and what do they do?

A

In transmmebrane or secreted from cells

Mainly repellent

Bind to plexin receptors

Class 4-7 = short range

Class 3 is long range

48
Q

What do Slits do?

A

Act as long range and repulsive cues by binding to Robo receptors and exist in 3 forms (1,2 and 3)

49
Q

What are Ephrin’ and Eph’s?

A

Ephrin receptors are transmembrane.

5 Eph’s bind to Ephrin A’s and 3 EphB’s bind to Ephin B’x.

Interactions mediate contact attractiona and repulsion of growth cones and axon branching

50
Q

What do Rho GTPases do?

A

Regulate activity of F-actin and MT asociated proteins - Rho GTPases regulate growth cone motility and turning

51
Q

What do Guanine nucleotide exchange factors (GEFs) do?

A

Activate Rho GTPases

52
Q

What do GTPase activing proteins (GAPs) do?

A

Inactivatge Rho GTPases

53
Q

What are the pre-crossing stages?

A
  1. Floor plate cells secrete Netrin (V-D grad), growth cone expresses DCC (a growth cone attractant
  2. Floor plate cells also secrete Shh (V-D grad), growth cones express Shh receptor Boc
  3. Roof plate cells secrete BMPs: growth cones express type 1 and type 2 BMP receptors
54
Q

What happens in the first part of floor plate crossing?

A
  • Floor plate secretes repulsive guidance cue Slit
  • Prior, during and after crossing growth cone expresses Slit receptors - Robo 1 and 2
  • Growth cone repulsion is prevented by co-expression of Robo-31 (Rig1) silencing Robo1/2 activity
    • after cross, Robo31 expression extinguished Slit repels growth cone and prevents re-crossing
55
Q

What happens in the second part of floor plate crossing?

A
  • Floor plate secretes repulsve guidance cue Sema3B
  • Prior to and during cross growth cones express Sem3B receptors plexin A and neuropilin 2
  • Growth cone repulsion is prevented as most plexin A is not expressed at growth cone surface
    • After crossing, Plexin A translocats to surface allowing Sem3B to prevetn growth cone re-crossing
56
Q

Whagt happens in the post crossing stage of the floor plate?

A
  • Ventral spinal cord has an anterior to posterior gradient of secreted morphogen Wnt
  • Growth cones express Wnt-receptor-frizzled-3
    • Floor plate derived Slit and Sem3B prevent growth cone turning towards the floor plate parallel cours
57
Q

Explain what is happening in the image at each of the embryonic days.

A
58
Q
A
59
Q

How have transgenic mice provided evidence for the role of guidance molecules in modline crossing of commisural neurons?

A

WT and Netrin -/- = dorsal commissural neurons cross the floor plate in Netrin -/- mice

WT and Robo3-/- = dorsal commissural axons don’t cross the floor plate in Robo3 -/-

Fundamental Neuroscience (19 decks)

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