Growth Cones

Question 1

What was the Weiss Resonance Theory?

Answer 1

neuronal autgrowth occurs randomly and occurs to all targets
+ elimination of non-functional connections

Question 2

What was the Sperry Chemoaffinity hypothesis?

Answer 2

DIrected and specified outgrowth ocurs through axons following individual identification tags

Question 3

what did Cajal discover?

Answer 3

used silver staining to discover the growth cone
proposed that the growth cone senses cues in the environment

Question 4

Explain neuronal polarity:

Answer 4

Dendrites receive info
- have mcirotubules but are more disorganised

axons carry info away
- have highly polarised microtubules (+ve toward growth cone)

Question 5

What are MAPs?

Answer 5

microtubule associated proteins
- determine how microtubules are crosslinked
axons express Tau
dendrites express MAP2

Question 6

How is polarity determined?

Answer 6

determined by neurite selection
one immature neurite becomes growth cone
- choice is stochastic

Question 7

How are microtubules stabilised?

Answer 7

Uncommited neurites contain dynamic MTs (tyrosinated)
Stabilised MTs (acetylated) are present in newly polarised axons
- can be artificially stabilised using Taxol

Question 8

Dynamic MTs….

Answer 8

are constantly polymerising and depolymerising
requires constant transition btwn GTP and GD bound states

Question 9

How does taxol work as an anti-cancer drug?

Answer 9

Locks microtubules into their stable state
in cancer cells this prevents depolymerisation of the mitotic spindle normally required for cell division

Question 10

What are lamella and filopoida?

Answer 10

parts of a growth cone
Lamella - actin bundles crosslinked into net
Filopodia - actin bundles are polarised to form larger bundles

Question 11

F-actin treadmilling…

Answer 11

F-actin subunits are added to the outside
flow back towards middle
Get chopped and released

Question 12

Steps of Growth cone reorganisation to turn:

Answer 12

1. The Growth cone comes in contact w/ an attractive cue
2. F-actin treadmilling slows and F-actin accumulates
3. F-actin accumulation:
   - Stabilises filopodium
   - Drags microtubules into the back of the filopodium - microtubule capture

Question 13

What 2 key components lead to filopodial extension and reorientation of the microtubule?

Answer 13

1. Molecular clutch is engaged + rearward actin treadmilling slows
2. Actmyosin based actin-tubulin link captures microtubules

Question 14

What is growth cone collapse?

Answer 14

if a sensory growth cone touches an axon - retreats
maintains contact but does not continue to grow in that direction
f-actin is destabilised

Question 15

What Rho GTPases are involved in axon growth?

Answer 15

Cdc42 - Filopodia
Rac - lamellipodia
RhoA - stress fibres

Question 16

How do these GTPAses regulate axon growth?

Answer 16

Rac and Cdc42 are positive regulators
Cdc42 is req for dendrite and axon formation
Rac is req for axon growth

RhoA is a negative regulator
required for collapse response

Question 17

are Gtpases instructive or permissive?

Answer 17

instructive
several guidance factor receptors either…
- bind + modulate Rho GTPases directly
- Bind GEFs or GAPs which regulate Rho GTPases

Question 18

What does semaphorin signalling alter?

Answer 18

RhoA/Rac balance
Plexins are receptors to semaphorins

Question 19

Semaphorins in flies….

Answer 19

expressed in certain muscles to guide innervation by motor neurons

change in relative lvls of plexinB, Rac and RhoA changes sensitivity of motor axons to semas.

Question 20

Semaphorin bound to plexins =

Answer 20

if…
more Rac than RhoA
= growth cone extension

less Rac than RhoA
= growth cone collapse

Question 21

Regulation of guidance signals via GEFs:

Answer 21

allows more specificity as there are many more + have more restricted expression

Ephrins signal through GEF called Ephexin
-regulated Cdc42, RhoA and Rac

Ephexin inhibits Cdc42 and Rac
and promoted RhoA

Question 22

How can you induce growth cone turning with calcium release?

Answer 22

EGTA/EDTA - cages Ca2+ (mops it up from solution)
Ca2+ is released when UV light breaks the bonds of a cage

Question 23

How do calcium transients regulate Rho GTPases?

Answer 23

Ryanodine induced calcium release from intracellular stores can attract growth cones
ryanodine activated Rac/Cdc42 and spresses RhoA

Question 24

What are the 5 key points of regulation of the actin cytoskeleton?

Answer 24

1. Filament disassembly - activated by cofilin
2. Branching - initiated by Arp2/3 in response to activation by WASP/WAVE proteins
3. Termination of branch extensions by capping proteins
4. Filament assembly - regulated by profilin and thymosin
5. Actomyosin contractility - essentially crosslinking btwn branched structures