Axon guidance 3

Question 1

What happens to commissural axons responsiveness to netrins after transit of the midline

Answer 1

They lose the responsiveness
Ectopic floor plate placed before the midline caused axons to divert to the ectopic site. When it was placed after the midline it had no effect

Question 2

What is the importance of the loss of response to netrin after crossing the midline

Answer 2

Comissural axons are able to continue past the midline without turning (hindbrain) -

Question 3

What happens to commissural fibres after crossing the floor plate in the spinal cord

Answer 3

Commissural fibres become sensitive to repellents (semaphorins/split) and insensitive to netrin once they have crossed the floor plate run anteriorly

Question 4

What is the role of slit, comm and robo in a wild type fly at the ventral midline

Answer 4

Slit is a secreted protein found at the ventral midline - acts as a chemo-repellent for axons once they have crossed the midline
Robo encodes a receptor for slit - High levels of robo found on axons as they extend longitudinally and low levels needed to allow the axon to cross the midline
Comm - only expressed in neurons that are required to cross the midline - switched off after the axon has crossed the midline

Question 5

What happens in a slit mutant

Answer 5

Axons make their way to the ventral midline - once they reach it there is no chemo-repellent action so they collapse into a single bundle along the midline

Question 6

What happens in a robo mutant

Answer 6

Robo 1 mutant - loses sensitivity to slit - robo 2,3 still function so there still is some sensitivity
Results in the axon continuously crossing and re-crossing the ventral midline

Question 7

What happens in the commissureless mutant

Answer 7

If comm isn’t present - the axon upregulates robo so the axon remains extending longitudinally and doesn’t cross the midline
If comm is overexpressed robo is lost giving rise to the same result as the robo mutant phenotype

Question 8

How is robo prevented from functioning before crossing the midline

Answer 8

comm encodes a trafficking protein that prevents robo protein from reaching the cell surface so there is no response to slit repulsion before crossing the midline

Question 9

What two things are required for a follower axon to be able to use a pioneer axon to reach its target

Answer 9

1. How the following axon is able to stay on the scaffold

2. How the follower axon is able to get off the scaffold when it has reached the target

Question 10

What is the cell adhesion molecule involved in the homophyllic binding of a follower axon to the pioneer

Answer 10

Fasciclin II in insects

Question 11

What occurs in Fasciclin II mutants in the ventral nerve cord longitudinal tracts

Answer 11

defasciculated axons - overexpression - novel fasciculation

Question 12

What is the importance of controlling fasciculation when motor nerves are innervating their targets

Answer 12

Overexpression of FasII leads to the axons bypassing their targets as they fail to defasciculate

Question 13

What proteins are able to cause defasciculation

Answer 13

BEAT proteins - disrupt the homophyllic action between the cell adhesion molecules expressed on both the axon and the fascicle

Question 14

What is the discrete targeting method of selection in drosophila and grasshoppers

Answer 14

Ablation of specific target muscle cells led to the failure relevant motor axons to leave the main motor trunk at appropriate branch points - suggests the axons are looking for specific labels on the target cells

Question 15

What protein mutant is essentially the same as ablating the target muscle cell

Answer 15

Netrin mutant - even though the muscle is there the axons do not make synapses with them
Ectopic netrin leads to axons innervating the wrong muscles

Question 16

What is the importance of Fasciclin 3

Answer 16

Expressed in specific muscles and in the motor axons that would normally innervate them
Ectopic expression leads Fas3-expressing axons to innervate new targets