18 - Adaptation in Axon Pathfinding Flashcards
What is the gradient of Sema3A?
High at the dendrites, low at the axons
How does Sema3A get into the cortex to be detected by the neurons?
It is secreted by cells
In terms of Sema3A, where do axons grow?
At lower concentrations of Sema3A (repellant)
True or false: the importance of BDNF gradients has been confirmed
False: it has not been confirmed genetically
True or false: the importance of Sema3A gradients has been confirmed
True: it is known to play an important role in axon growth
If a petri dish has stripes of BDNF, where will the dendrites and axon be?
The dendrites will be away from the stripes, and the axon will be on the stripes
If a petri dish has stripes of Sema3A, where will the dendrites and axons be?
The dendrites will be on the stripes, and the axon will be away from the stripes
Why is a gradient of extracellular cues important?
There would be no reason to grow towards / away something (guided by concentration of factors)
What would happen if there was a constant concentration of extracellular cues?
There would not be proper growth, since the cells would not be guided on where to go (based on gradients)
What happens if Sema3A is overexpressed in cortical neurons?
The gradient will be abolished, which will impact neuron growth
How can a molecule gradient be abolished?
Overexpression or deletion of the molecule
What conclusion can be drawn from the fact that axon initiation starts polarization, and that there is only one axon?
The axon must send signals to inhibit the other neurites from becoming an axon
What molecule acts as a negative crosstalk in cell polarization?
cGMP
What is needed for the initial breaking of the symmetry?
A neurite achieves a threshold of signaling that leads to axon formation
What does the axon do to other neurties?
It sends negative signals to prevent them from reaching the threshold signaling to become an axon
What happens in neurites inhibited by the axon?
They undergo other signaling to become dendrites (prevent threshold signaling of becoming an axon)
True or false: one neurite can communicate with other neurites
True: this leads to only one axon being generated
What happens if cAMP is locally elevated in one neurite?
It is locally decreased in other neurites
What drives axon and dendrite formation?
Separate signaling pathways that engage in negative communication
Which molecules have opposing roles in regulating axon and dendrite formation?
cAMP and cGMP
What properties does cGMP have?
Water soluble (intracellular)
What does GC do?
Create cGMP from GTP
What enzyme creates cGMP?
Guanylyl cyclase (GC)
What enzyme breaks down cGMP?
A phosphodiesterase (PDE)
Where are ACs found?
In the membrane (transmembrane)
Where are GCs found?
In the membrane (transmembrane), or soluble (in the cytoplasm)
What regulates AC?
G-proteins
What regulates GC?
NO (nitric oxide)
What does NO stand for?
Nitric oxide
What is the major source of cGMP in the brain?
Soluble GC (sGC)
What does sGC stand for?
Soluble guanylyl cyclase
What are the possible downstream targets of cGMP?
PKG, ion channels, and cGMP-PDEs
What does PKG stand for?
Protein kinase G
What cellular events use cGMP?
Neuronal development, vascular smooth muscle relaxation, and vision
True or false: cAMP and cGMP need to be modified to cross the cell membrane
True: they are highly water soluble, not lipid soluble
True or false: NO needs to be modified to cross the cell membrane
False: it is uncharged and nonpolar, so it can easily pass through the cell membrane
What are the characteristics of NO?
Small, uncharged, and nonpolar
What enzyme produces NO?
NOS (nitric oxide synthase)
What does NOS stand for?
Nitric oxide synthase
What does NOS do?
Converts L-arginine into NO
What is nNOS?
A neural specific NOS
How stable is NO?
Very unstable (only active for a few seconds)
What is the consequence of NO being highly unstable?
It is highly transient and leads to local effects
What type of signaling would be ideal for NO?
Transient paracrine and autocrine signaling
What is the downstream target of NO?
sGC
What is the structure of sGC?
A heterodimer with a beta regulatory domain, and an alpha domain
What is the structure of the regulatory domain of sGC?
A heme functional group (iron)
What is the function of iron in sGC?
It acts as a high affinity binding site for NO
What happens if no NO is bound to the regulatory domain of sGC?
It is inactivated (no cGMP is produced)
True or false: NO can only bind to sGC at the iron
False: it has two modes of binding (high affinity and low affinity)
When does NO bind to the high affinity binding site?
At low concentrations
When does NO bind to the low affinity binding site?
At high concentrations
Where is the high affinity binding site?
At the heme group
What is the activity of sGC at low concentrations of NO?
Partial activation (only high affinity binding)
What is the activity of sGC at high concentrations of NP?
Full activation (low affinity binding too)
Why is sGC partially activated at low concentrations of NO?
Only binding at the high affinity binding site
Why is sGC fully activated at high concentrations of NO?
There is also binding at the low affinity binding site
What happens if cAMP is presented to a neuron?
The axon will grow in the cAMP, and the dendrites will grow outside of the cAMP
What happens if cGMP is presented to a neuron?
The axon will grow outside of the cGMP, and the dendrites will grow in the cGMP
How can the cAMP and cGMP pathway be arranged to antagonize each other?
- They have opposite actions on common downstream targets
2. Reciprocal regulation (through activation of PDEs)
What molecule increases cGMP levels?
Sema3A
What is the effect of LKB1 when forskolin and Sema3A are added?
Decreased phosphorylation
Why is there decreased phosphorylation of LKB1 when forskolin and Sema3A are added?
The cGMP inhibits the cAMP pathway
What is downstream of PKG?
Dendrite formation
What are the levels of cyclic nucleotides in the axon?
High cAMP and low cGMP
What are the levels of cyclic nucleotides in the dendrite?
High cGMP and low cAMP
What happens if PKA is inhibited in a neurite?
It will most likely differentiate into an axon
True or false: inhibiting PKA has the same phenotype as overexpressing cGMP
True: both lead to dendrite formation
What determines whether a neurite will be an axon or a dendrite?
The relative ratio of determinants from the cAMP and cGMP pathway
What is the significance of sGC having two modes of activation?
This creates a basal level of cGMP, which can be used in signaling
True or false: biological processes are usually at zero
False: they usually have a low, basal condition
What determines the attraction and repulsion of the axon in axonal pathfinding?
The cAMP/cGMP ratio
When the cAMP/cGMP ratio is high, what is the phenotype of the axon?
Attraction (turn towards it)
When the cAMP/cGMP ratio is low, what is the phenotype of the axon?
Repulsion (turns away from it)
How do guidance cues mediate axon pathfinding?
They regulate the cAMP/cGMP ratio
What does a positive cue do to the neuron (intracellularly)?
Increases the levels of cAMP
What does a negative cue do to the neuron (intracellularly)?
Increases the levels of cGMP
What happens if PKA is inhibited and netrin is presented to the axon?
It will shift away from netrin
