L12 - Axon Guidance III Flashcards
Axons reprogram when intermediate targets are encountered
Sensitivity of axons to floor plate (netrins) changes after midline is crossed
Sensitivity of axons to floor plate (netrins) changes after midline is crossed - hindbrain
Commissural axons are able to continue past floor plate without turning
Sensitivity of axons to floor plate (netrins) changes after midline is crossed - spinal cord
Commissural axons turn after crossing the floor plate
Open book preparation of tissue cut from embryo before axons have begun to extend method
Fluorescent lipophilic dyes
- Absorbed into and highlight cell membranes
- Put on either side of the fixed tissue to monitor axon growth
Open book preparation of tissue cut from embryo before axons have begun to extend results
Axons exposed to ectopic floor plate before reaching midline - respond by turning
Axons exposed to ectopic floor plate after crossing midline - no longer respond
After crossing midline, commissural axons become sensitive to?
Inhibitory molecules in the floor plate
Semaphorins and slits
Also expressed in ventral spinal cord creating a channel through which commissural axons grow
After crossing midline, commissural axons become sensitive to? - results
Initially axons are sensitive to netrin but not inhibitors
After crossing the floor plate become sensitive to inhibitors
Midline glial cells express?
Diffusible attractants - netrins
Cell surface repellents - slits
Growth cone sensitivities get reprogrammed
Some axons cross forming commissures, and then turn to join pathways formed by axons that have not crossed
Levels of slit receptor determine?
Whether axons can cross the midline
What is robo?
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
In robo mutants
Slit no longer detected
Axons go back and forth across midline forming Roundabouts of axons
What is comm?
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
In comm mutants
Robo is expressed at high levels on axons that normally cross midline
Now extend their axons longitudinally
If Comm’s expression is forced in all neurons?
Robo protein lost
Phenotype just like the Robo mutant
- Comm controls Robo
Robo1 - vertebrate homologue of Robo
Expressed on commissural axon
Expressed both before and after crossing
Robo3 - vertebrate homologue of Robo
Expressed only in pre-crossing fibres
Blocks Robo1 signalling until the midline is crossed
What is pioneer navigation?
Helps establish axon scaffold which follower axons can follow
How do axons stay on and get on the scaffold?
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
Fas II
If expressed in cells that normally do not adhere, Fas II can cause aggregation
Fas II controls fasciculation in?
Flies in the ventral nerve cord longitudinal tracts
Fas II fasciculation mutants
Defasciculated axons
Fas II fasciculation overexpression
Novel fasciculations
Fas II and other CAM adhesion can also be regulated by?
Expression of other proteins
E.g. BEAT - interfere with CAM-mediated adhesion
Fas II defasciculation overexpression
By pass phenotype
Motor axons fail to defasciculate and miss their targets
What are the two types of target selection?
Discrete targets - cellular in DNS
Topographic Maps - multicellular
What suggests axons are looking for specific labels on their targets?
In Grasshopper and Drosophila - ablation of specific target muscles leads to failure of motor axons to leave main motor trunk at appropriate branch points
Insect muscles carry molecules that together may constitute muscle address labels
Netrin
Fasciclin 3
Netrin
Diffusible chemoattractant
Loss of Netrin is like ablating muscles - axons wander and do not make synapses
Ectopic Netrin - axons innervate wrong muscles
Fasciclin 3
Homophilic adhesion molecules
Expressed in specific muscles and in motor axons that innervate them
Ectopic Fas3 – axons innervate new targets
What are topographic maps?
Neighbouring neurons send axons to neighbouring sites in their target to maintain target topology
E.g. retinotectal system
How is target topology maintained?
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
What does a stripe assay show?
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
Inhibitory factor in posterior tectum is?
Two ephrins expressed in a gradient
- Posterior – high
- Anterior – low
Eph receptor for ephrins A2 and A5 is expressed in the retina in?
A counter gradient
- Temporal - high
- Nasal – low
Ephrin A2 and A5 knockout mice
Temporal neurons project their axons into posterior tectum and topographic map is disordered
Non-permissive repellant factors can be used instructively
They can direct growth cones to specific places to form topographic maps
