strategies of axon guidance

Question 1

What is an example of how axons reprogram when intermediate targets are encountered?

Answer 1

• After transit of midline, commissural axons lose netrin responsiveness
• Experiment with an ectopic floor plate, initially attracted but once crossed it isnt, to prevent the turning back
• gain response to repellents as midline is crossed
• Inhibitory molecules found to be semaphorins and slits
• As axons go through an intermediate target their sensitivities can change

Question 2

What are netrins and slits?

Answer 2

Netrins are expressed by midline glial cells and act as attractants.
Slits are inhibitory signals expressed on the cell surface to prevent recrossing.

Question 3

What were the key mutants studied in flies regarding commissural fibres crossing?

Answer 3

1. Roundabout (Robo): Mutants lack functional Robo, a receptor for Slits, causing axons to fail to respond to inhibition and circle repeatedly around the midline.
2. Commissureless (Comm): Mutants lack Comm, a protein that prevents Robo from reaching the axon surface before midline crossing. Without Comm, Robo is active too early, and axons are repelled, failing to cross the midline.

Question 4

In the case of commissural fibres, what else changes upon crossing the midline?

Answer 4

• TAG-1 and L1 expression (proteins)

Question 5

What controls the upregulation of Robo when the midline has been crossed?

Answer 5

• Slit is expressed by floor plate (FP) cells and acts as a repellent.
• Before crossing the midline, Robo receptors are kept inside the cell to prevent premature repulsion by Slit.
• After crossing, Slit binding triggers Robo trafficking to the cell surface.
• Robo on the surface makes the axon sensitive to Slit, preventing recrossing of the midline.
• Loss of the trafficking pathway prevents Robo from reaching the surface, trapping commissural axons in the FP.
• If this doesnt happen both sides of the brain arent connected meaning sensory and motor control cannot connect

Question 6

Key before and after midline comparison:

Answer 6

Before midline:
PRRG4/Com prevents Robo cycling to the axon so the netrin can bind to its receptor to cause attraction

After midline:
Activation of Robo by slit upregulates axonal Robo and silences Netrin receptor signalling

Key point: Contact with high slit in the midline is what changes the growth cone response in the FP