L12 - Axon Guidance III

Question 1

Axons reprogram when intermediate targets are encountered

Answer 1

Sensitivity of axons to floor plate (netrins) changes after midline is crossed

Question 2

Sensitivity of axons to floor plate (netrins) changes after midline is crossed - hindbrain

Answer 2

Commissural axons are able to continue past floor plate without turning

Question 3

Sensitivity of axons to floor plate (netrins) changes after midline is crossed - spinal cord

Answer 3

Commissural axons turn after crossing the floor plate

Question 4

Open book preparation of tissue cut from embryo before axons have begun to extend method

Answer 4

Fluorescent lipophilic dyes

• Absorbed into and highlight cell membranes
• Put on either side of the fixed tissue to monitor axon growth

Question 5

Open book preparation of tissue cut from embryo before axons have begun to extend results

Answer 5

Axons exposed to ectopic floor plate before reaching midline - respond by turning
Axons exposed to ectopic floor plate after crossing midline - no longer respond

Question 6

After crossing midline, commissural axons become sensitive to?

Answer 6

Inhibitory molecules in the floor plate
Semaphorins and slits
Also expressed in ventral spinal cord creating a channel through which commissural axons grow

Question 7

After crossing midline, commissural axons become sensitive to? - results

Answer 7

Initially axons are sensitive to netrin but not inhibitors

After crossing the floor plate become sensitive to inhibitors

Question 8

Midline glial cells express?

Answer 8

Diffusible attractants - netrins

Cell surface repellents - slits

Question 9

Growth cone sensitivities get reprogrammed

Answer 9

Some axons cross forming commissures, and then turn to join pathways formed by axons that have not crossed

Question 10

Levels of slit receptor determine?

Answer 10

Whether axons can cross the midline

Question 11

What is robo?

Answer 11

Encodes receptor for Slit
Expressed at high levels on axons that don’t cross midline
Commissural axons initially express low levels but high levels after they cross midline

Question 12

In robo mutants

Answer 12

Slit no longer detected

Axons go back and forth across midline forming Roundabouts of axons

Question 13

What is comm?

Answer 13

Expressed only in neurons that normally cross the midline
Encodes trafficking protein that prevents Robo reaching cell surface
- Growth cone cannot receive Slit inhibitory signals before midline X-ing

Question 14

In comm mutants

Answer 14

Robo is expressed at high levels on axons that normally cross midline
Now extend their axons longitudinally

Question 15

If Comm’s expression is forced in all neurons?

Answer 15

Robo protein lost
Phenotype just like the Robo mutant
- Comm controls Robo

Question 16

Robo1 - vertebrate homologue of Robo

Answer 16

Expressed on commissural axon

Expressed both before and after crossing

Question 17

Robo3 - vertebrate homologue of Robo

Answer 17

Expressed only in pre-crossing fibres

Blocks Robo1 signalling until the midline is crossed

Question 18

What is pioneer navigation?

Answer 18

Helps establish axon scaffold which follower axons can follow

Question 19

How do axons stay on and get on the scaffold?

Answer 19

Both involve controlling fasiculation
Involves homophilic binding by cell adhesion molecules (CAMs)
- E.g. Fasciclin II in insects
Homophilic interactions can bind two cell surfaces together

Question 20

Fas II

Answer 20

If expressed in cells that normally do not adhere, Fas II can cause aggregation

Question 21

Fas II controls fasciculation in?

Answer 21

Flies in the ventral nerve cord longitudinal tracts

Question 22

Fas II fasciculation mutants

Answer 22

Defasciculated axons

Question 23

Fas II fasciculation overexpression

Answer 23

Novel fasciculations

Question 24

Fas II and other CAM adhesion can also be regulated by?

Answer 24

Expression of other proteins

E.g. BEAT - interfere with CAM-mediated adhesion

Question 25

Fas II defasciculation overexpression

Answer 25

By pass phenotype

Motor axons fail to defasciculate and miss their targets

Question 26

What are the two types of target selection?

Answer 26

Discrete targets - cellular in DNS

Topographic Maps - multicellular

Question 27

What suggests axons are looking for specific labels on their targets?

Answer 27

In Grasshopper and Drosophila - ablation of specific target muscles leads to failure of motor axons to leave main motor trunk at appropriate branch points

Question 28

Insect muscles carry molecules that together may constitute muscle address labels

Answer 28

Netrin

Fasciclin 3

Question 29

Netrin

Answer 29

Diffusible chemoattractant
Loss of Netrin is like ablating muscles - axons wander and do not make synapses
Ectopic Netrin - axons innervate wrong muscles

Question 30

Fasciclin 3

Answer 30

Homophilic adhesion molecules
Expressed in specific muscles and in motor axons that innervate them
Ectopic Fas3 – axons innervate new targets

Question 31

What are topographic maps?

Answer 31

Neighbouring neurons send axons to neighbouring sites in their target to maintain target topology
E.g. retinotectal system

Question 32

How is target topology maintained?

Answer 32

Each axon has a unique label complementary to a unique label in target
= Not enough genes to make that many labels
Co-ordinate system, encoded by gradients of signalling molecules, stamps a latitude and longitude onto cells of target
- Read by complementary gradients of receptors expressed in retinal ganglion cells

Question 33

What does a stripe assay show?

Answer 33

Shows cells from posterior tectum make a non-permissive factor that repels temporal retinal axons
Due to temporal axons avoiding a repellant factor in posterior stripes because:
- Activity is abolished by heat treatment of posterior but not anterior membranes
- Posterior membranes cause temporal growth cones to collapse

Question 34

Inhibitory factor in posterior tectum is?

Answer 34

Two ephrins expressed in a gradient

• Posterior – high
• Anterior – low

Question 35

Eph receptor for ephrins A2 and A5 is expressed in the retina in?

Answer 35

A counter gradient

• Temporal - high
• Nasal – low

Question 36

Ephrin A2 and A5 knockout mice

Answer 36

Temporal neurons project their axons into posterior tectum and topographic map is disordered

Question 37

Non-permissive repellant factors can be used instructively

Answer 37

They can direct growth cones to specific places to form topographic maps