Lecture 11 Axon guidance 1 Flashcards
How many connections in brain and neurons
10^11 neurons with 10^3 connections so 10^14 connections
What are the two hypothesises for axon guidance?
WEISS: RESONANCE THEORY
SPERRY: CHEMOAFFINITY HYPOTHESIS
What was WEISS’ theory?
Stochastic (random) and diffuse neuronal outgrowth occurs to all targets followed by elimination of non-functional connections
What was SPERRY’s theory?
Directed and specific outgrowth occurs through axons following “individual identification tags” carried by the “cells and fibers” of the embryo
They both did tests on the visual system. What is the tectum?
tectum does preliminary visual processing, and controls eye movement
What is another name for tectum?
Superior colliculus
How are the axons between the eyes and tectum organised?
- anterior retina (nasal) sends axons into posterior tectum
- posterior retina (temporal) send axons into anterior tectum
- same organisation on ventral/dorsal plane
Experiments were carried out on the tectum in
Newts then frogs
Why is the visual system organised like this?
so we see real images (correct orientation)
If SPERRY was correct, cutting the posterior retinal axons i.e. removing temporal retina and optic nerve, what would happen?
the anterior retinal neurons (nasal axons) would go to their normal location
If WEISS was correct, cutting the posterior retinal axons and optic nerve, what would happen?
there would be a diffusion of axons then elimination
Who was correct - Sperry or Weiss
Sperry
Does Sperry experiment show that axons are guided during development?
no, it was done on an ADULT frog, so it shows regeneration of axons show axon guidance
Cells were ablated and so some cells could remain allowing axons to follow the tracts
What other experiments did Sperry carry out
Extreme temporal and nasal ablation - axons went inbetween showing gradation of info varies dependent on the retinal axon ablated
Hence label directs growth –> FINE MAP
What would you expect to see if Weiss was right in embryo experiments and what was actually seen?
Expect random patterns of axons
Yet highly sterotyped organised reproducible
How do we know motor neurons are specifically guided to their targets in chick embryos?
In EARLY stage CHICK before MN have grown out
- cut and replace, or reverse, segment of the neural tube before motor axons grow
- when replaced in same orientation, motor neuron axons can still navigate to their normal targets
- when replaced in reverse orientation, the axons still find their way to targets = suggests axons actively navigate
What are guidance cues?
Factors in environment that axons use to find their correct target
To learn more about axon growth, experiments were done in insects. Why?
- simple NS
- ease of observation/manipulation as large and dev outside body
- in large insects like grasshoppers, individual cells could be ablated
What was found from axon growth experiments on insects
- Axon pathways are stereotyped from embryo to embryo and segment to segment
- they tacked and mapped axons which allowed them to located cues
What did AB staining show in grasshoppers
Ventral nerve cord
Longitudinal axon fascicles
Commissures (Axon crossing)
What was done to see where cues were located? What was found?
ablated nearby neurons to see if it affect axon pathway
- it was found that most continued as normal
- it was found that the G axon stalled in absence of the P axon as G axon looking for cues on P axon
Reproducibility of axon behaviour in ablation suggests
Label in eco
When do pathways change
When a specific axon is encountered
Ablation studies showed that NOT
due to lack of axon on which to extend
due to decrease in number of axons
What hypothesis do ablation studies support
Labelled pathway hypothesis
This process was called labelled pathway hypothesis. What does this suggest?
- Axons can selectively fasciculate with other axons
- Axon surfaces carry labels or cues
- Different axon growth cones express different sets of receptors for such cues
- Early axons (pioneers) form an axon scaffold on which later axons (followers) can extend
- Establishes axon surfaces as one potential source of guidance cue
How come the first axons know where to go if it is thought the axons use each other to guide themselves?
the environment that is thought to be featureless has molecular differences that guide axons
- there are TFs that are already set up by the embryo (previous lectures) which the axons use to guide themselves
e.g. SHh and BMP in spinal cord on DV
Describe axon scaffolds in vertebrates
Subplate neurons visual cortex
Project from cortex to thalamus prior to innervation of cortex by LGN. if ablate part of subplate early on, before axons extend LGN innervation fails here
Give example in grasshopper of how pioneer cells follow sterotyped paths
limb - Pioneer tibial 1 (Ti1) growth cone turns at limb boundary as approaches CX1
Ablation of CX1 means GC stalls at other side of boundary
Other cells in the grasshopper pathway like CX1 are called
Stepping stone or guidepost cells
How are pioneer axons guided in the forebrain, hindbrain and spinal cord?
- FOREBRAIN: Axons follow boundaries of domains of patterning gene expression
- HINDBRAIN: Axons follow boundaries of rhombomeres
- SPINAL CORD: Axons are attracted to, and follow boundaries of the floor plate
Guidance of pioneer axons shows
Axons AND many embryo cell types contain guidance cells
Guidance cues can be and examples
+/-
e. g. ablation - GC stall - attractive force lost
e. g. Ti1 GC avoid limb boundary - inhibitory
What are the 4 forces of axon guidance?
Contact repulson/attraction
Chemoattractant/repulsion
How do neurons know where to put their growth cones?
- polarisation - one end is different to other in shape and function: due to different neurites
- > axons - carry info away
- > dendrites - receive incoming info
How do axon microtubules differ to those of dendrites?
Axon MT highly organised as + GC and polarised
Dendrite MT less ordered and mixed orientation
Why is there a difference in organisation of MT and axon/dendrite
Localisation of different types of MT associated proteins (MAPS)
MT associated proteins for axons
TAU
MT associated proteins for dendrites
MAP2
What determines polarity
Neurite selection in hippocampus
Define neurite
Any projection from cell body if immature neurite = axon or dendrite
What marks where axons forming
GFP as +end directed kinesins Kif 1 mark
How is a neurite selected to become a GC
Random after different neurites tried out
Neurite to axon selection
microtubule (MT) stabilisation:
- competition between neurites to stabilise MTs as removal casues another neurite to be selected
- some kind of feedback loop to prevent other neurites being selected
Difference between nascent and stabilised MT
Nascent = tyrosinated MT - dynamic MT Stabilised = acetylated MT - only in newly polarised axons
Taxol
Artifically stabilised MT causing neurite selection
