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Fundamental Neuroscience > Axon growth and guidance > Flashcards

Axon growth and guidance Flashcards

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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?

Study These Flashcards
A

Microtubule association

25
What needs to happen for the molecular clutch to form?
Growth cone needs to encounter an adhesive substrate
26
What are the four stages of axon growth?
1. Adhesive substrate encounter 2. Protusion 3. Engorgement 4. Consolidation
27
What happens in the first stage of axon growth?
*_Adhesive substrate encounter_*: * Formation of **molecular clutch** in filopodia * **Slow** local **retrograde flow** of F-actin
28
What happens in the second stage of axon growth?
*_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
What happens in the 3rd stage of axon growth?
*_Engorgement:_* * Dynamic central domain MTs guided into extended TZ by actin arcs (causes extended central domain)
30
What happens in the 4th stage of axon growth?
*_Consolidation:_* * New axon shaft froms behind advancing CD due to MT stabilization
31
What do actin nucleators do?
Form G-actin trimers prior to F-actn assembly
32
What do F-actin capping proteins do?
**Inhibit F-actin** assembly at **positive end** of filaments and **inhibit G-actin cleavage** at negayive end
33
What do F-actin polymerisation factors do?
Promote F-actin assembly and prevent capping proteins from binding to positive end of F-actin
34
What do F-actin severing proteins do?
Remove ADP G-actin from positive end of F actin and cleave actin filaments
35
What do monomer binding proteins do?
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
What do motor proteins do?
Mediate retrograde flow of F-actin and retrograde and anterograde transport of vesicles and macrmolecules
37
What do F-actin bidning proteins do?
Bundling of F-actin filaments, F-actin cross-linking in mesh like networks and link F-actin to plasma membrane
38
What are the proteins that regulate growth cone motility and growth cone turning? (6 marks)
* F-actin capping proteins * F-actin polymerisation factors * F-actin severing proteins * Monomer binding proteins * Motor proteins * F-actin binding filaments
39
What do plus end tracking proteins do?
Bind to the postiive ends of dynamic, polymerising MTs to stabilise newly added α/ ß tubulin dimers e.g. APC, APC2, CLASP
40
What do microtubule stabilising proteins do?
Bind to existing microtublues to stabilise their structure e.g. MAP1B, TAU
41
What do MT sestablisng proteins do?
Induce MT depolymerisation (catastrophe) counteracting microtubule polymerisation/ elongation
42
What do MT severing proteins do?
Induce microtubule cleavage important in axon branching
43
What do motor proteins do?
Retrograde (dynein) and anterograde (kinesin 5) transport of vesicles and macromolecules alonfg axon
44
What are the four types of molecular guidance cues? (8 marks)
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
What do adhesive substrate bound cues consist of?
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
What are netrins and what do they do?
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
What are semaphorins and what do they do?
In transmmebrane or secreted from cells Mainly repellent Bind to **plexin receptors** Class 4-7 = short range Class 3 is long range
48
What do Slits do?
Act as long range and repulsive cues by binding to Robo receptors and exist in 3 forms (1,2 and 3)
49
What are Ephrin' and Eph's?
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
What do Rho GTPases do?
Regulate activity of F-actin and MT asociated proteins - Rho GTPases regulate growth cone motility and turning
51
What do Guanine nucleotide exchange factors (GEFs) do?
Activate Rho GTPases
52
What do GTPase activing proteins (GAPs) do?
Inactivatge Rho GTPases
53
What are the pre-crossing stages?
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
What happens in the first part of floor plate crossing?
* 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
What happens in the second part of floor plate crossing?
* 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
Whagt happens in the post crossing stage of the floor plate?
* 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
Explain what is happening in the image at each of the embryonic days.
58
59
How have transgenic mice provided evidence for the role of guidance molecules in modline crossing of commisural neurons?
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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