15 - Signaling Mechanisms Underlying Axon Guidance I Flashcards
What question can be asked regarding redundancy?
Do we have redundancy, or are some proteins critical for functioning
True or false: there are neuropathologies associated with LKB1
False: there are no neuropathies associated with LKB1
What is STRAD?
A cofactor for LKB1
What is the cofactor of LKB1?
STRAD
In basal conditions, where is LKB1 located?
In the nucleus
In basal conditions, where is STRAD located?
In the cytoplasm and in the nucleus
What do STRAD and MO25 do to LKB1?
- Phosphorylate it at Ser431 to activate it
2. Localize LKB1 into the cytoplasm to function
What would happen to a cell without STRAD?
LKB1 would be localized in the nucleus
What functions does LKB1 have in the nucleus?
Unknown, but it cannot mediate its downstream targets for cell polarization
True or false: there are neuropathologies associated with STRAD
True: PMSE is associated with a truncated STRAD gene
What are the symptoms of PMSE?
Psychomotor disability, epilepsy, and early childhood mortality
Why does PMSE lead to neuronal problems?
The truncated STRAD cannot localize LKB1 into the nucleus for proper axon formation
How come there are no pathologies associated with LKB1?
It is a critical enzyme, and thus its deletion is lethal (embryonic death)
How come there are pathologies associated with STRAD?
It is important, but not critical, for axon development, so people can still survive (other mechanisms)
What happens to STRAD in PMSE?
It gets truncated
What is the importance of crosstalk?
Regulation, specificity, redundancy, and amplification
What are the components of a signaling pathway (for an intricate and regulated pathway)?
On/off processes with time delays
What is a feed forward mechanism?
A signaling mechanism originates at one upstream regulator, and converges on one function
True or false: for a feed forward mechanism, the paths must be the same
False: the paths (signaling determinants) can be different, but the start and end points have to be the same
True or false: for a feed forward mechanism, the start and end points must be the same
True: the paths (signaling determinants) can be different, but the start and end points have to be the same
What is the advantage of a feed forward mechanism?
It is a relentless signaling to activate a critical function
True or false: the paths in a feed forward mechanism must converge at the same time
False: there can be time delays between the different paths, as long as they converge on the same function
What is the difference between a feedback and a feed forward?
A feedback loops back onto an upstream regulator, while a feed forward moves forward towards a downstream function
What is coupled to a positive feed forward mechanism?
Negative regulation (with a time delay)
What is coherent feed forward?
When the two branches have the overall same sign
What is incoherent feed forward?
When the two branches have the opposite signs overall
True or false: a coherent feed forward can be positive or negative
True: as long as the overall messages of each branch are the same, it is a coherent feed forward
True or false: a positive coherent feed forward can only have positive linkages
False: only the overall message of the branch needs to be positive (A –| B –| C = A –> C)
True or false: a negative coherent feed forward can only have negative linkages
False: only the overall message of the branch needs to be negative (A –> B –| C = A –| C)
True or false: a coherent feed forward must have the same linkages
False: they only need to communicate the same overall message (A –| B –| C = A –> C)
True or false: an incoherent feed forward must have different linkages
False: they only need to communicate different overall messages (A –| B –| C = A –> C)
What is the relationship between the cAMP pathway and the PI3K pathway in polarization?
A coherent feed forward
What does GSK3b stand for?
Glycogen synthase kinase 3 beta
What does GSK3b do?
Regulates glycogen synthesis, and mediates polarization (and other functions)
When GSK3b is phosphorylated, what state is it in?
Inactive
When GSK3b is not phosphorylated, what state is it in?
Active
What is the natural state of GSK3b in the cell?
Constitutively active
What enzyme inactivates GSK3b?
Akt
What enzyme sits in the middle of the crosstalk between the cAMP and PI3K pathway?
GSK3b
Which amino acid regulates GSK3b function?
Ser9
What state does GSK3b need to be in to promote axon development?
Inactive (phosphorylated) state
What is a downstream effector of GSK3b?
CRMP-2
What is the upstream regulator of CRMP-2?
GSK3b
What does CRMP-2 do?
Regulates microtubule formation
When CRMP-2 is phosphorylated, what state is it in?
Inactive
When CRMP-2 is not phosphorylated, what state is it in?
Active
What state does CRMP-2 need to be in to promote axon development
Active (non phosphorylated) state
What does CRMP-2 stand for?
Collapsin-response mediator protein 2
What is one of the most downstream effectors of axon formation and development?
Microtubule regulation
What is the growth cone?
The tip of the neurites
What is the tip of the neurite called?
The growth cone
What happens at the growth cone?
Signaling for axonal development occurs, with constant shape changes
True or false: the growth cone is a static structure
False: it is highly dynamic
Where are the signaling pathways for axonal development centralized?
In the growth cones
What is the structure of the growth cone?
Rich in actin and microtubule cytoskeleton
What is the majority of the growth cone comprised of?
Actin cytoskeleton
What does the actin cytoskeleton do in the growth cone?
Monitors and responds to the environment
What does the microtubule do in the neurites?
Builds the stem of the neurites
True or false: the actin and microtubule cytoskeletons in the growth cone do not interact
False: the microtubule can invade the actin network and regulate the shape of the growth cone
What does the growth cone respond to?
Gradients of extracellular cues
How does the growth cone respond to extracellular cues?
Through its receptors
Where are the receptors to respond to extracellular cues (for axon formation) found?
In the growth cone
What is the structure of the microtubules?
A hollow cylinder
What are the monomers of the microtubules?
GTP-bound alpha and beta tubulin
How many filaments comprise a microtubule?
13 filaments
True or false: the microtubules are static structures
False: they are highly dynamic (can grow and shrink)
True or false: microtubules are symmetrical
False: they have directionality (plus and minus end)
What happens at the plus end of microtubules?
Adds tubulin subunits
What happens at the minus end of microtubules?
It gets degraded
What do plus end stabilizer proteins do?
Bind to the plus end of the microtubule and stabilize it (no synthesis)
How does CRMP-2 work?
It binds to tubulin monomers and incorporates them into the microtubule
Why does CRMP-2 need to be not phosphorylated to mediate microtubule formation?
It has a lower affinity for tubulin when it is phosphorylated
When is CRMP-2 inactived?
When GSK3b is activated
When is CRMP-2 activated?
When GSK3b is inactivated
What happens to the cell when GSK3b is inhibited?
Multiple axons form
What happens to the cell when GSK3b is constitutively activated?
No axons form
What is needed in the axon (in terms of GSK3b and CRMP-2) to promote axonal development?
Unphosphorylated (activated) CRMP-2 and phosphorylated (inactivated) GSK3b
