Growth Cones Flashcards
What was the Weiss Resonance Theory?
neuronal autgrowth occurs randomly and occurs to all targets
+ elimination of non-functional connections
What was the Sperry Chemoaffinity hypothesis?
DIrected and specified outgrowth ocurs through axons following individual identification tags
what did Cajal discover?
used silver staining to discover the growth cone
proposed that the growth cone senses cues in the environment
Explain neuronal polarity:
Dendrites receive info
- have mcirotubules but are more disorganised
axons carry info away
- have highly polarised microtubules (+ve toward growth cone)
What are MAPs?
microtubule associated proteins
- determine how microtubules are crosslinked
axons express Tau
dendrites express MAP2
How is polarity determined?
determined by neurite selection
one immature neurite becomes growth cone
- choice is stochastic
How are microtubules stabilised?
Uncommited neurites contain dynamic MTs (tyrosinated)
Stabilised MTs (acetylated) are present in newly polarised axons
- can be artificially stabilised using Taxol
Dynamic MTs….
are constantly polymerising and depolymerising
requires constant transition btwn GTP and GD bound states
How does taxol work as an anti-cancer drug?
Locks microtubules into their stable state
in cancer cells this prevents depolymerisation of the mitotic spindle normally required for cell division
What are lamella and filopoida?
parts of a growth cone
Lamella - actin bundles crosslinked into net
Filopodia - actin bundles are polarised to form larger bundles
F-actin treadmilling…
F-actin subunits are added to the outside
flow back towards middle
Get chopped and released
Steps of Growth cone reorganisation to turn:
- The Growth cone comes in contact w/ an attractive cue
- F-actin treadmilling slows and F-actin accumulates
- F-actin accumulation:
- Stabilises filopodium
- Drags microtubules into the back of the filopodium - microtubule capture
What 2 key components lead to filopodial extension and reorientation of the microtubule?
- Molecular clutch is engaged + rearward actin treadmilling slows
- Actmyosin based actin-tubulin link captures microtubules
What is growth cone collapse?
if a sensory growth cone touches an axon - retreats
maintains contact but does not continue to grow in that direction
f-actin is destabilised
What Rho GTPases are involved in axon growth?
Cdc42 - Filopodia
Rac - lamellipodia
RhoA - stress fibres
How do these GTPAses regulate axon growth?
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
are Gtpases instructive or permissive?
instructive
several guidance factor receptors either…
- bind + modulate Rho GTPases directly
- Bind GEFs or GAPs which regulate Rho GTPases
What does semaphorin signalling alter?
RhoA/Rac balance
Plexins are receptors to semaphorins
Semaphorins in flies….
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.
Semaphorin bound to plexins =
if…
more Rac than RhoA
= growth cone extension
less Rac than RhoA
= growth cone collapse
Regulation of guidance signals via GEFs:
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
How can you induce growth cone turning with calcium release?
EGTA/EDTA - cages Ca2+ (mops it up from solution)
Ca2+ is released when UV light breaks the bonds of a cage
How do calcium transients regulate Rho GTPases?
Ryanodine induced calcium release from intracellular stores can attract growth cones
ryanodine activated Rac/Cdc42 and spresses RhoA
What are the 5 key points of regulation of the actin cytoskeleton?
- Filament disassembly - activated by cofilin
- Branching - initiated by Arp2/3 in response to activation by WASP/WAVE proteins
- Termination of branch extensions by capping proteins
- Filament assembly - regulated by profilin and thymosin
- Actomyosin contractility - essentially crosslinking btwn branched structures
