Lecture 21- Axon guidance IV Flashcards
What is axon guidance important for?
-nervous system connectivity- precise wiring of the nervous system is important for neural function -axon’s finding their targets is incredibly important -the nerves have to find the target in a complex environment and very far e.g. the cyatic nerve -very important for Neuron migration – the cell will often follow the direction of the leading neurite to find the correct location (eg. Cortical neuron migration)
How are axons guided?
-guided to their target by environmental cues -Growing axons recognize various cues on the surface of other axons and cells -and respond to diffusible molecules such as Morphogens (these may be produced quite far away, also called ligands, diffusible molecules) - and use intermediate targets (cues on ther route -also cues on the target itself)
What are the environmental guidance cues sensed by on the axon?
-sensed by the growth cone -the growth cone is the one that senses most of the environmental guidance cues -they use receptors that are highly concentrated in the growth cone area, that activates signalling pathways resulting in cytoskeletal changes
What do the growth cones used to recognise guidance cues?
-environmental cues are detected by receptors on the growth cone -Ligand binding to receptors on the growth cone activates downstream signalling pathways which alter the actin cytoskeleton and change the direction of axon growth -cue= morphogens= ligand -generally affects Rho Rac pathway
What sort of cues can there be?
-Environmental cues may be attractive or repulsive -short range= usually contact dependent -long range= produced long way away, diffusible substances
How does the reaction to long range cues work?
-growth cones guided by cues released from a distance source (chemoattraction/ chemorepulsion) -Chemoattractants and repellents act in a gradient -the axon moves from low to high concentration (usually- if attractive) -growth cone can tell the difference in conc in 1 in a 1000 -attractive cue= there will be high conc close to the origin= -gradient of expression
What must happen for chemotaxis to occur?
- for chemotaxis to occur the growth cone must be positioned so that one side is exposed to a higher level of factor (to change direction) -e.g NGF (nerve growth factor) attracts axons, can see when in culture, always go to the source
What is the example of long range attractive cue in mice?
- Cues secreted from cells in the maxillary pad (whiskers)attract axons from the trigeminal ganglion
- tissue surrounding the maxillary pad has no affect on the axons
- the maxillary pad produces the neurotrophins, BDNF and NT-3 which attract the axons (two proteins that belong to the same family ad NGF)
- explant assay= take chunk of tissue from the origin and target
- take neighbouring tissue as well
Is repulsion as important as attraction in axon guidance?
-yes
What can contact with axons or cells result in in axons?
-contact with other axons or cells can repulse growth cones -contact dependent repulsion =causes the cytoskeleton to pull back quite quickly -e.g, sensory growth cone repelled by motor axon
Are axons growing from tissue explants repelled by the same tissues in vivo as in vitro?
-yes
What is the example of a repulsive cue?
- olfactory bulb neurons= grow along the septum, but do not grow into it
- something secerted by the septum to repulse them
- they head in the opposite direction to the septum
What is the example of a distant repellant cue?
-Growing axons may be repelled by distant factors (chemorepellents) -slit proteins are chemorepellents -act to repel axons
What are the main categories or guidance molecules?
- the main families
- ligands= environmental cues
- receptors that they will activate
- blue= the main function the pathway has -some are mainly involved in attraction and some in repulsion
- The Growth factors= acttraction
- Slit= repulsion -some can do both attraction and repulsion= netrin and ephrin
What is the example of an attractive guidance molecule?
- netrin signalling
- First identified in C.elegans (Unc6)
- in mammals, 3 secreted netrins (Netrin1,2,3) and 2 membrane bound netrins (Netrin G1 and G2)
- Receptors for secreted netrins: DCC, neogenin, Unc5A-5D and DSCAM (Unc40 in C. Elegans; Frazzled in Drosophila)
- Receptors for membrane bound netrins: NGL-1 and NGL-2(these are in the membrane)
- homologues in all species= in mammals multiple netrins secreted
- contact dependent molecules
- multiple receptors= initiate different UNC5 A- UNC5 B, UNC5 C , UNC5 D
- looking at the red ones
What is the downstream signalling pathway of netrin?
- the netrin:DCC signalling pathways
- netrin= binds to DCC -causes axon formation, cytoskeletal remodelling (axon guidance)
- transcription resulting in long term protein synthesis, microtubule dynamics
What are the dorsal commissural neurons?
- dorsal commissural neurons= dorsal column of the spinal cord where the cell bodies are, need to send the axons ventrally and to cross over the floor plate
- then head down anteriorly top form a white matter tract that runs longitudinally along the spinal cord
What was the experiment using dorsal commissural neurons?
- attraction of dorsal commissural axon guidance in the spinal cord, attracted by netrin-1 to the floorplate
- netrin is expresed by the floor plate region -growth cone expresses DCC (receptor for Netrin1)
- makes the axons grow towards the floor plate
- they also performed an explant assay= to make sure that it was netrin that was involved
- this confirmed that it is specifically netrin acting on the axons
What is the Eph-Ephrin signalling like?
- activation of Eph receptor results in receptor phosphorylation, Rho GTPase activation and remodelling of the actin cytoskeleton (forward signalling)
- ephrins are also able to signal (reverse signalling) (kinase activation and other signalling pathways)
- both signalling events can happen simultaneously (can result in a reaction on both cells)
