Tactics and Strategies of Axon guidance

Question 1

What are the 4 tactics of axon guidance?

Answer 1

Haptotaxic - Contact attraction
- Contact repulsion

Chemotaxic - Chemoattraction
- Chemorepulsion

Question 2

How far can netrin diffuse?

Answer 2

normally expressed in floor plate
may diffuse as far as 0.5mm (half the size of spinal chord at that time)

Question 3

How to measure gradients?

Answer 3

Temporal detection - change in conc/time
cell must compare amount of ligand at diff time points

Spatial detection - Change in concentration across the cell

Question 3

What does accuracy of orientation depend on?

Answer 3

• Absolute concentration of chemoattractant
  (most accurate response closest to Kd (dissociation constant) of the receptor)
• steepness of the gradient

Question 3

What is the suggested mechanisms for amplification?

Answer 3

clustering of receptor/ signalling components in regions where receptors are activated
- transport components from other parts of the cell

Question 4

What are chemoattractants detected by?

Answer 4

GPCRs
- activate PI3K
- which phosphorylated PIP2 into PIP3
provide docking sites for proteins containing pleckstrin homology domains
- creates a localised signalling domain on the membrane

Question 4

What is PI3K antagonised by?

Answer 4

PTEN phosphatase

Question 5

What do PI3K inhibitors do?

Answer 5

disrupt growth cone turning

Question 6

The frog axon turning assay showed….. at the growth cone

Answer 6

alternative attractive and repulsive turning
- this suggests cycles of desensitization and sensitisation of the growth cone to netrin

Question 7

pretreatment of axons to netrin led to

Answer 7

no growth cone response to netrin

Question 7

What is an example of axon reprogramming?

Answer 7

sensitivity of axons to the floor plate (netrins) changes after the midline is crossed
(in the hindbrain)

Question 7

What are the inhibitory molecules present in the ventral spinal cord?

Answer 7

semaphorins and slits

Question 8

What happens when axons cross the floor plate?

Answer 8

Their sensitivities switch
become insensitive to netrin but sensitive to inhibitors

Question 9

What are slits?

Answer 9

Slits are large secreted proteins shown to act in axon guidance
prohibit growth cone advance an acts as guard rails

Question 10

What are semaphorins?

Answer 10

diverse family of cel surface and cytosolic proteins
potent activity in promoting growth cone collapse

Question 11

What are the receptors for semaphorins?

Answer 11

plexins ad neuropilins

Question 12

What is Robo?

Answer 12

Roundabout (Robo) encodes the membrane receptor for the midline repellant slit

Question 13

what is the robo mutant?

Answer 13

robo mutant has no longitudinals
the axon crosses the midline glial cells over and over

Question 14

What is commissureless?

Answer 14

encodes a trafficking protein that prevents robo from reaching the cell surface

Question 15

What are the comm mutants?

Answer 15

no commissures
everything goes longitudinally

Question 16

What is the vertebrate homologue of Comm?

Answer 16

PRRG1

Question 17

how does cell surface expression change after crossing the midline in vertebrates?

Answer 17

TAG-1 -> L1

Question 18

what does slit induce?

Answer 18

robo trafficking to axons

Question 19

What occurs in axons before the midline?

Answer 19

PRPG4/ Comm prevents Robo from cycling to the axon
netrin can bind to its receptor DCC

Question 20

what occurs after the midline?

Answer 20

activation of Robo by Slit upregulates axonal robo (Robo affects the RhoA/Cdc42 balance)
this silences Dcc signalling