16 - Signaling Mechanisms Underlying Axon Guidance II Flashcards
What is the common effector of the cAMP and PI3K pathway?
GSK-3b
What is the benefit of a feed forward mechanism?
It creates a very powerful and forceful signaling mechanism
Why is a feed forward mechanism needed for axonal development?
It is a prolonged task, which requires relentless signaling to achieve
How does the PI3K pathway interact with GSK-3b?
Akt can inhibit GSK-3b through phosphorylation
How does the cAMP pathway interact with GSK-3b?
PKA can inhibit GSK-3b through phosphorylation
For a western blot of phosphorylated GSK-3b, what is the blot with cAMP and why?
Heavy blot, because cAMP activates PKA, which phosphorylates GSK-3b
For a western blot of phosphorylated GSK-3b, what is the blot with cAMP and PKA inhibitor and why?
No blot, because PKA is not active to phosphorylate GSK-3b
For a western blot of phosphorylated GSK-3b, what is the blot with forskolin and why?
Heavy blot, because forskolin activates AC, which leads to cAMP and PKA being active, thus phosphorylating GSK-3b
For a western blot of phosphorylated GSK-3b, what is the blot with forskolin and PKA inhibitor and why?
No blot, because PKA is not active to phosphorylate GSK-3b
True or false: the PKA pathway and the PI3K pathway only converge at GSK-3b
False: these two pathways have many levels of integration
How does aPKC interact with GSK-3b?
aPKC can phosphorylate GSK-3b and inhibit it
How does LKB1 interact with the PI3K pathway?
LKB1 can associate with aPKC and lead to cross activation
True or false: LKB1 can phosphorylate GSK-3b
True: this has been seen in some studies
Besides the PI3K pathway, what pathway is Akt an upstream regulator for?
The mTOR pathway
What does the mTOR pathway do (in terms of axonal development)?
Promote translation of PAR-1
How does Akt interact with the cAMP pathway?
Through the mTOR pathway, it can activate translation of PAR-1 (the downstream regulator of LKB1)
What three things are needed for a coherent feed forward mechanism?
- One upstream activator
- Diverging branches with the same “message”
- Convergence on one downstream target
True or false: the cAMP and PI3K pathway can be activated by the same extracellular factor
True: they can both be activated by BDNF
True or false: the cAMP and PI3K pathway can be activated by different extracellular factors
True: these pathway can run parallel to one another with different extracellular cues
Which extracellular factor can activate both the cAMP and the PI3K pathways?
BDNF
What pathways does BDNF promote?
Both the cAMP and the PI3K pathways
What does BDNF stand for?
Brain derived neurotrophic factor
What family of proteins does BDNF belong to?
Neurotrophin family of growth factors
What is BDNF related to?
NGF
What does NGF stand for?
Nerve growth factor
What functions does BDNF mediate?
Survival, growth differentiation, and synaptic function of neurons
What is the receptor for BDNF?
TrkB
What is TrkB?
An RTK for BDNF
Where are neurotrophins found?
In the brain and in the peripheral nervous system
How does BDNF activate the PI3K pathway?
Through the phosphorylation of its RTK TrkB
How does BDNF activate the cAMP pathway?
Through the inhibition of PDE4 (increase cAMP levels)
What is needed for axons to respond to BDNF in vitro?
LKB1, PKA, and PI3K
How can we test that axon formation in response to BDNF depends on LKB1, PKA, and PI3K?
Through genetic and/or pharmacological manipulations
If TrkB is phosphorylated, what is the effect on the axon?
It will promote axon development
Where does most of the axonal development happen?
At the growth cones
What occupies the “palm like” structure of the growth cone?
The actin cytoskeleton
Which part of the axon responds to the extracellular environment?
The growth cone
What do most of the signaling pathways in axonal development converge on?
Regulation of the cytoskeleton
What does MAP stand for?
Microtubule associated protein
What are some examples of MAPs?
CRMP-2, Tau, and MAP2
What do MAPs do?
Regulate the growth of the microtubule cytoskeleton
What type of protein is the Tau protein?
A MAP
What is the downstream target of PAR-1?
Tau protein
What is the upstream regulator of Tau protein?
PAR-1
What does Tau protein do?
Stabilize the assembly of the axon microtubule filaments
When is Tau protein activated?
When it is phosphorylated by PAR-1
What protein is similar to Tau protein?
CRMP-2
What protein is similar to CRMP-2?
Tau protein
What happens when Tau protein is phosphorylated?
It can associate with the tubulin monomers and regulate the microtubules
True or false: MAPs are only axonal specific
False: there can also be dendritic specific MAPs
What is an example of an axonal specific MAP protein?
CRMP-2, Tau protein
What is an example of a dendritic specific MAP protein?
MAP2
What is MAP2?
A dendrite specific MAP
What does MAP2 do?
Mediate microtubule assembly, stabilization, and bundling
How do the microtubules interact with the actin cytoskeleton in the growth cone?
They stabilize and penetrate the actin network
What needs to happen for axons to experience “bursts of growth”?
Microtubules must penetrate the actin network
What is needed to allow the microtubules to invade the actin cytoskeleton in the growth cone?
Actin destabilization (depolymerization)
In terms of the cytoskeleton, what is needed to allow for the growth of the growth cones?
Actin instability, and microtubule stability
Why is actin instability needed for the growth cones to grow?
Allow for microtubule invasion
Why is microtubule stability needed for the growth cones to grow?
Allow for structure to move forward
What does colchicine do?
Causes microtubule instability
What does taxol do?
Causes microtubule stability
What does cytochalasin do?
Causes actin filament instability
What does phalloidin do?
Causes actin filament stability
Which drug causes microtubule instability?
Colchicine
Which drug causes microtubule stability?
Taxol
Which drug causes actin filament instability?
Cytochalasin
Which drug causes actin filament stability?
Phalloidin
Which drugs (that target the cytoskeleton) would promote axon formation?
Taxol and cytochalasin
Which drugs (that target the cytoskeleton) would not promote axon formation?
Colchicine and phalloidin
If taxol or cytochalasin is presented to the whole cell, what happens?
It will develop multiple axons
If taxol or cytochalasin is presented to a specific neurite, what happens?
That neurite will become the axon
True or false: taxol could potentially rescue the function of LKB1
True: taxol stabilizes the microtubule cytoskeleton, which is a very downstream regulator
What is a good target for the downstream regulation of axonal development?
The cytoskeleton
Why is the cytoskeleton a good target for axonal development?
It is one of the most downstream targets that mediates axonal formation
What is axonal guidance (or path finding)?
How the axon grows to a specific target
True or false: axonal guidance is random
False: it is highly specific
True or false: axonal guidance can only create straight lines
False: sharp turns can also be created through axonal guidance
What is the purpose of axonal guidance?
Have the axon reach its innervation target (synpase)
What happens when an axon reaches its innervation target?
It creates a synapse
How does axonal guidance work?
It goes through intermediate proteins until it reaches its innervation target
What is needed to choose a route to go from A to B?
A map
What information does a map provide?
Where to go (positive cues) and where not to go (negative cues)
What “map” is present in axonal guidance?
Molecular cues
What do the intermediate proteins do in axonal guidance?
They secrete positive and negative cues to guide the axon
In axonal guidance, what do positive cues do?
Signal for the axon to grow towards it
In axonal guidance, what do negative cues do?
Signal for the axon to grow away from it
How do negative cues cause the axon to grow away from it?
It causes cytoskeleton collapse
What cues does the innervation target secrete?
Positive cues
What mediates the turns in the growth cone?
The microtubule cytoskeleton
What does a positive cue do to the microtubule cytoskeleton?
It causes the filaments to grow in that direction
What does a negative cue do to the microtubule cytoskeleton?
It causes the filaments to collapse (depolymerization)
What is the net result of a positive and negative cue being presented together in axonal guidance?
A turn is produced
What happens to arrays of microtubules in axonal guidance?
Some microtubules can collapse, while others can grow (based on the spatial arrangement of these positive and negative cues)
What are the positive cues in axonal guidance?
Chemoattractants
What are the negative cues in axonal guidance?
Chemoretractants
How does the growth cones detect the chemoattractants and chemoretractants?
Through specific cell surface receptors
What is the affect of the chemoattractants and the chemoretractants on the cell (in axonal guidance)?
Activation or inhibition (respectively) of the microtubule cytoskeleton
