Axon Guidance 3 Flashcards
What is the retinotectal projection pathway and why is it important
The route taken by retinal ganglion cell (RGC) axons from their source in the retina to their targets in the brain is probably the best studied projection pathway in the body, and provides an excellent example of how axon guidance cues co-operate to achieve accurate and topographical axon guidance
Where do the majority of retinotectal projection pathway axons terminate
Tectum - in fish
Superior colliculus - in mammals
What is the key site for the retinal axons
Lateral geniculate nucleus (LGN) in the thalmus
What is the role of the lateral geniculate nucleus
where post-synaptic axons relay retinal information to the primary visual cortex.
What is important about eye position in animals and the retinotectal projection pathway make-up
Non-mammals (frog) - lateral facing eyes with no crossover in visual field.
Mammals - forward-facing eyes so crossover in visual field.
Non-mammal pathways - retinal axon projections all cross to the other side of the head at the optic chiasm, so that all axons from the left eye project to the right side of the brain and those from the right eye all project to the left side of the brain.
Mammalian pathways - a proportion of axons from the mammalian ventrotemporal retina do not cross the chiasm but project ipsilaterally to the brain on the same side of the head. Thus each side of the brain receives visual information from both eyes.
How does ventrotemporal cross over produce better vision compared to the complete crossover of axon fibres shown in non-mammals
This provides mammals generally with better visual acuity, particularly in terms of perspective.
Explain the growth pattern of the retinotectal projection pathway
First they have to grow across the surface of the retina to the optic nerve head (ONH), at which point they turn away from the surface and extend out of the back of the ye into the optic nerve. This takes them to the ventral surface of the diencephalons where they enter the brain and negotiate the optic chiasm. They then grow along the optic tract to the tectum (or colliculus) where they terminate.
How do neighbouring RGCs interact
The RGCs have opposite but complementary terminal positions in the tectum/colliculus -
This means that a RGC immediately dorsal to another in the retina will project its axon to a position immediately ventral to it in the tectum
How do these RCG axons know the direct path to take
Brittis and silver 1995
Unknown but it’s been purposed that - a ring of inhibitory chondroitin sulphate proteoglycan (CSPG) recedes peripherally, starting just when the first retinal axons begin to extend just dorsal to the centre of the retina.
Neuroepithelial cells express axon promoting ECM molecules
(laminin) within this CSPG ring to continue to allow the RCGs to grow.
Later RGC axons may piggyback along the pioneer axons since they all express a number of different homophilic cell adhesion molecules, but the pathfinding mystery remains to be confirmed/discovered.
However, this phenomenon has only been observed in one study in the mouse and has not been found in other organisms, so many scientists are sceptical that such a mechanism exists
What was shown in mice that had undergone netrin knockout
WIDER READING - Deiner 1997
retinal axons have trouble growing through the Optic nerve head (ONH).
Resulting in a thinner optic nerve, due to fewer axons making it out of the eye.
Results in optic nerve hypoplasia. Shows taht DCC guides RCG axons out the optic disc locally rather than long range.
What has been shown in vitro regarding netrin and optic growth cones
In vitro, growth cone turning assays have shown that axons newly formed by retinal neurons are attracted to a netrin gradient, but it is not thought that netrin can diffuse far across the retinal surface, so it probably acts locally at the ONH.
What does laminin do to netrin and therefore growth cones
But netrin does not attract retinal axons in the presence of laminin; in fact, concentrated laminin induces growth cones to be repelled by netrin
Important - laminin is not inhibitory to axon growth, it just induces netrin to drop cAMP and prevent axon growth.
What does laminin do to cAMP
Prevents an increase in cAMP activity.
Where is laminin expressed
On the vitreal surface of the retina but not in the optic nerve head.
Why is laminin only present on the vitreal surface
Axons grow in presence of cAMP so stay away from vitreal surface and maintain on ONH.
It could also be the case, however, that the shape of the ONH, which essentially forms a hole at the back of the eye, means that there is only one way for axons to go, but that would not explain why so many axons do not grow through the ONH when the netrin gene is knocked out.
What happens to netrin after retinal axons go through the ONH and why
retinal axons’ attraction for netrin is switched off - allows the axons to grow on through the optic nerve rather than turn back towards the ONH where the netrin concentration is high.
What is expressed in the middle of the optic chiasm in mammals
EphrinB family - help with ipsilateral repulsion.
This occurs as these axon fibers express EphB receptors.
Is ephrinB expressed in non-mammals
No as all their fibers cross. But if its expression is induced, axons are deflected away from the chiasm midline. Slits are also expressed at the chiasm, but in a pattern that appears to define the chiasm shape rather than at the midline across which axons grow.
What cytoskeleton factor is said to prevent early crossing of these ipsilateral fibres
Plump et al 2002
Slits = shown through knockout
Doesnt seperate subpopulations of axons into their respective tracts as one would expect it to be expressed in the midline of the optic chiasm.
What role does L1 have in the optic chaism
Mason and sretavan 1997
Radial glial cells are found as a edge shape within the midline of the chiasm - express L1, a cell adhesion molecule that is permissive for growth and therefore sort the optic chaism axons and also expresses CD44 and ephrins to prevent undesired cross over by inhibiting axon growth in that direction.
What is the optic tract
the conduit through which retinal axons enter the tectum.
What is the role of the optic tract and how was this discovered
Local cues to guide axons in the tract - tissue transplant in frogs caused a rotation of the optic tract through 90 degrees prior to the arrival of the first growing axons results in those axons making the same 90 degrees turn.
This shows the tectum isnt a chemoattractive source as should’ve continued to go towards it no matter what position. It is likely, then, that the axons require physical contacts in the tract to grow in the right direction. Thought to be with FGF signalling as by blocking this, it fails to grow into the tectum
Why do images received by the retina appear upside down
Diffraction.
It therefore has to be re-inverted on the tectum, so the brain can recognise the image the right way up.
What did sperry discovered in the 1960s regarding rotating frogs eyes (180 degrees)
When the frogs were then presented with food they could not grab it with their tongues. If the food was placed high, the frogs aimed low, and vice versa. The frogs were effectively seeing their world upside down.
What did bongoeffer’s stripe assay discover about axon guidance specificity in the tectal tissue
Took tectum and cut in half - anterior and posterior half and stripped individual membranes into their respective anterior and posterior lanes -
Then takes strip of retina and lay it perpendicularly to the axon lane membranes.
On temporal - grew on anterior strip
On nasal - grew on posterior and anterior (no preference)
Suggests that temporal region has inhibitory molecule to stop posterior strip growth.
What molecule causes the posterior growth inhibition in temporal neurons
Braisted 1997
Ephrins - receptors found to be expressed in RGC gradients across the retina.
Ephrin ligands are found in complementary gradients within the tectum/colliculus (higher in posterior so inhibits it sooner, lower in anterior so promotes it causing deeper tectum growth)
(Drescher 2002)
What role does netrin have in the tectium
As cAMP is low at this late stage, netrin is expressed by the posterior tectum, preventing retinal axons innervating the posterior tectum
What role does sema 3A have in the tectum
Expressed in the posterior tectum like netrin.
Retinal axons become sensitive to sema3A with maturity - helping tectum mapping.
In summary, where do ephrin A, ephrin B and netrin prevent axon growth in the tectum
Netrin-1 - posterior
Ephrin B - ventral/inferior
Ephrin A - anterior/posterior
Overall - this allows visual images to be turned back around 180 degrees.
What role do glial cells have in axon guidance
WIDER READING - Dotal et al 2010
Corpus callosum formation in vertebrates - premature glia cells migrate to the ependymal zones (thin neuroepithelial lining of the cerebral ventricles) to form a structure the first axons of the corpus callosum use to extend.
How did the ventral nerve cord of drosophilia help understand axon guidance in the corpus callosum
WIDER READING - Sun et al 2000
The currently accepted model is that Slit, produced by midline cells, repels axons from the midline via Robo receptors. Ipsilaterally projecting (non-crossing) axons always have Robo receptors on their surface, while commissural axons have very little or no Robo on their surface, allowing them to be attracted to the midline by Netrins.
After crossing, however, Robo receptors are strongly upregulated on the axon in these commissural fibres, which allows Robo-mediated repulsion to overcome attraction to the midline and continue crossing over.
What did stein show regarding netrin organisation
WIDER READING 2001
attracted, repressed, or silenced in the presence of Netrin activated DCC receptor -
Slit activated Robo receptor, causes a silencing of Netrin’s attractive potential through the DCC receptor. While growth cones expressing UNC-5 receptor, respond in a repulsive manner to Netrin-DCC activation.
This shows the complex picture regarding axon guidance.
How do growth cones become guided axons
WIDER READING - LOWERY
Have pools of mRNA to code for receptors adn intracellular signalling proteins involved in cytoskeleton remodelling
What did FGFR inhibition show in mice regarding RGC axons
WIDER READING - McFarlane 1996
FGFR inhibited which caused a 40% in axon advancement speed but appropriate path until the tectum where they bypassed it rather than entering it.
Demonstrate that FGFR signaling is needed for establishing the initial connectivity between these neurons
