Wiring of visual system

Retinal ganglion cells must:

1. Exit the eye.  

 

2. Join the optic nerve  

 

3. Cross (not all) the optic chiasm (midline)  

 

4. Form terminal branches with targets:  

   • E.g., lateral geniculate nucleus + superior colliculus  

    

5. Terminate at specific layers and positions within these targets according to RGC subtype and retinal location.  

The Retinotopic Map:

• The image in the retina is upside down, but then re-inverted in the SC / tectum.  

 

Models for axon targeting: functional selection / predetermination:  

• Predetermination - connections are pre-determined.  

 

• Functional Selection - axons form connections, before pruning and formation of final connectional pattern.  

Evidence for axon targeting mechanism: - Pre-determination:  

• Sperry's optic nerve regeneration experiment:  

 

• In amphibians, optic nerve will regrow after it has been cut, and make new connections.  

• This experiment cut the optic nerve, rotated the eye 180°, allowed for regeneration before testing vision.  

 

• Functional selection would result in restoration of normal fusion.  

• Predetermination would result in 180° rotated visual field.  

• Alternative/intermediate mechanisms would result in blurred vision.  

 The newt swam downwards instead of upwards towards prey like normally.  

• This supports the rotation of the visual field i.e., predetermination mechanism

Chemo-affinity Hypothesis:

These experiments led to Sperry forming the Chemo-affinity hypothesis"seems from these results that cell + fibres of the brain + spinal cord must carry some kind of individual tags, cytochemical in nature, by which theyre distinguished from each other…"  

 

 

"each axon linking only with certain neurons to which it becomes selectively attached to by specific chemical affinity."  

 

 

• Diagram shows the specificity of retinal --> tectum connections:  

  • Dorsal of retina --> ventral of tectum. Etc… 

Molecular mechanisms of axon guidance:

Above shows repulsive or attractive chemical guidance cues.  

• E.g., chemo-repulsion from Semaphorin.  

• E.g., chemo-attraction from netrins. 

• E,g., contact repulsion from Ephrin. 

• E.g., contact repulsion from L-CAMs. 

Netrins present a highly conserved axon guidance signal:  

• Crossing midline of spinal cord as an example.  

  • Netrin secreted from the floorplate, leads to attraction of the commissural fibre axons to floorplate.  

  • Experiment places dorsal spinal cord next to floorplate segment (secreting Netrin).  

    • Observed axonal growth towards floorplate.  

 

Signalling at the growth cone:

Growth cone dynamics: constantly remodelling cytoskeleton via actin treadmilling.  

• RhoGTPase : acts on actin/myosin cytoskeleton.  

  • GEFs hydrolyse GTP --> axon extension.  

  • GAPs restore GTP --> retraction of axon. 

Support for the chemo-affinity hypothesis:  

• The nasal retinal cells project to the posterior side of tectum.  

• The temporal retinal cells project to the anterior side of tectum.  

 

• This experiment used a micropatterning:  

  • Rows of posterior tectum membrane. 

  • Rows of anterior tectum membrane.  

 

• It was observed that the axons from temporal RGCs grew on stripes containing anterior tectal membrane.  

• The nasal RGC axons grew indiscriminately. 

Expression gradients of Ephrin and Eph Receptor: 

• (A) shows that EphA3 receptor mRNA is distributed in a gradient of temporal (more) --> nasal (less) of the retina.  

  • Ephrin-A5 = distributed in a posterior --> anterior gradient in tectum.  

 

• (B) shows retino-tectal mapping: 

  • Temporal axons express lots of EphA3 --> targets anterior of tectum because expresses less Ephrin-A5 (repels A3). 

  • Nasal axons express less Eph3A --> can target posterior of tectum that expresses more Ephrin-A5. (less repulsion) 

Genetic Validation of the Ephrin-A gradient control of RGC wiring:

EphrinA2/A5 -/- mice had both Nasal and Temporal axons scattered around the anterior/posterior axis of the superior colliculus, as opposed to the expected: T-->A // N-->P. 

RGC targeting position is determined by Eph expression levels:

• (A) shows results from a wildtype vs genetic knock-in mice. 40% of wildtype RGCs express Isl2, this expression is increased in the knock-in. 

  • Isl2+ cells express EphA3.  

  • Therefore knock-in mice expressed more EphA3.  

Mice RGCs with higher EphA3 levels (Temporal retina) target more anterior areas of Superior Colliculus, while RGCs from nasal retina with lower EphA3 --> target posterior.  

• The higher EphA3 expression (Isl2+) leads to pushing back (more posterior) of Isl2- RGCs.  

Counter-gradients and bidirectional signaling of ephrin-A and Epha 

• (A) shows that both ephrin-A and EphA are expressed as counter gradients, i.e., inversely proportionate.  

 

• (B) shows schematic of forward + reverse Ephrin-A/EphA signalling.  

  • Forward = ephrin-A acts as ligand to send signal to EphA receptor expressing growth cone.  

  • Reverse = EphA acts as a ligand to send signal to ephrin-A expressing growth cone.