amplification + adaptation Flashcards
What is meant by haptotaxic and chemotaxic?
haptotaxic = contact attraction/repulsion
chemotaxic = chemoattraction/ chemorepulsion
What is the minimum amount a chemoattractant should be able to travel?
- At the time most experiments were done the spinal cord is about a mm, suggesting the chemoattractant should be able to work at least half a mm
- They can extend over quite a long distance but is this the limit?
What are the possible explanations for commissural axons turning rostrally after crossing the floor plate?
- either a repellent down forcing it to brain or an attractive anterior cue
- long range diffusible cue or a short range non-diffusible cue
How do we distinguish between a diffusible/non-diffusible cue for the C axons crossing the floor plate?
Diffusible cue: The cue would spread away from the source, making it difficult to maintain a stable gradient. A spinal cord section in a dish would show the cue diffusing, disrupting the gradient over time.
Non-diffusible cue: The cue remains localized and stable, without diffusing away. The axons would respond to the cue within a limited range, maintaining a steady gradient.
How can we tell if it is a diffusible attractant or repellent cue?
Diffusible attractant: As the cue spreads, the anterior-most axons may encounter a weaker gradient and turn abnormally.
Diffusible repellent: As the cue spreads, the posterior-most axons are more likely to encounter the highest concentration and be repelled, affecting their direction.
Is the commisural axon cue attractant or repellent?
Found to be an attractant molecule
So its a long range , attractant cue
Travelling for 3mm!
What is the cue? and how was it studied?
- Wnt4 is expressed in the FP in an A-P gradient, in the ‘right place at the right time’
-Take the molecule and ectopically put it in A or P and it will turn the post crossing axons in either direction
KO experiment to see if its NECESSARY
Could do a conditional KO but at the time they just KO the receptor called Frizzled3, loss of frizzled 3 stops the cells from moving upwards and instead they are just confused
Explain what determines the accuracy of orientation
- What % of the cells are oriented towards the chemoattractant
- Bigger diameter cells orient better - comparing over greater distance
- The growth cones are extremely sensitive and need to detect tiny differences on receptor occupation on each side
How is amplification carried out in the axon growth cones?
- Reduce other signals –> ramp key signal
- taking away extracellular parts that would be used for other signals, to stop them from happening so you can only ‘hear’ the important signal
- generates movement in that direction
What is a signalling pathway that is an example of amplificiation?
- Chemoattractant binds to GPCR, activating PI3-kinase.
- PI3-kinase converts PIP2 to PIP3 (a second messenger).
- PIP3 activates downstream signaling molecules (e.g., Akt), leading to cytoskeletal changes and growth cone movement.
- This creates a cascade effect, amplifying the initial signal and influencing a larger area of the growth cone.
- The pathway also involves a positive feedback loop, enhancing the signal and ensuring a strong directional response.
How is adaptation seen in the experiment with frogs and netrin?
- Frog axons are placed in a dish with netrin delivered via a pipette.
- Netrin creates a gradient visible with a fluorescent marker.
- Over time, axons show fluctuating trajectories (zig-zag direction during navigation), which suggests cycles of sensitization (response to the gradient) and desensitization (temporary unresponsiveness as receptors adapt to high netrin levels).
- Axons climb the gradient (desensitization occurs at higher concentrations), then reset sensitivity and repeat the process.
What is meant by the pre-exposure effect?
- If netrin is added to the dish at a low concentration beforehand, the axons initially do not respond to the pipette (as receptors are desensitized).
- Over time, growth cones recover sensitivity and begin responding to the pipette, forming a straighter trajectory toward it.
What are the two gradient types?
- Point Source Gradient:
- A localized source (e.g., the pipette) creates an exponential gradient that decreases with distance from the source.
- Substrate-bound Gradient:
Created by molecules bound to a surface (e.g., ephrin ligands on a substrate interacting with Eph receptors).
What do sensitisation and desensitisation cycles allow axons to do?
navigate shallow gradients despite receptor saturation
