lec 11-13. axon guidance Flashcards
Weiss resonance theory
random neuronal outgrowth to all targets followed by elimination of non-functional connections
Sperry chemoaffinity hypothesis
axons grow directly and specifically to individual ID tags carried by cells of the embryo
how sperry tested his hypothesis
cut the optic nerve and removed the temporal retina. nasal neurons grew specifically to the posterior end -> sperrys theory was right
why insects are used for axon guidance experiments
- have simple nervous systems
- embryos are easy to observe and manipulate
- can ablate larger cells like in grasshopper
labelled pathway hypothesis
axons express different receptors on their growth cones and can selectively fasciculate with other axons that carry labels.
axon scaffolds
pioneers form an axon scaffold that followers can extend on (which are important in vertebrates too!)
example of axon scaffolds in vertebrates
subplate neurons which project into the thalamus provide a scaffold for LGN neurons to reach the cortex
grasshopper embryo limb experiment
- asking how pioneers find their way
Ti1 (pioneer) growth cone makes specific turns at limb border as it approaches Cx1 cell. ablation of guidepost cells (Cx1 and other cells in pathway) lead to Ti1 stalling. - there must be molecular differences in the environment , and guidances cues on cell types other than just axons
four types of guidance cues
chemoattraction
chemorepulsion
contact attraction
contact repulsion
how does a neuron know which end the growth cone should be
axons express Tau, and dendrites express MAP2, giving the neuron polarity, which is determined by neurite selection
neurite selection
appears to be random after different neurites are ‘tried out’.
microtubule stabalisation is critical for axon formation
new axons have ____ microtubules
polarised axons have ____ microtubules
new axons - tyrosinated dynamic microtubules
polarised axons - acetylated stabalised microtubules
lamellopodia
made up of F-actin. actin bundles are cross-linked into net
filopodium
made up of F-actin. acting bundles are polarised to form larger bundles
resting growth cone movement
F-actin treadmills and tubulin sporadically gets dragged into filopodium
growth cone movement when in contact with attractive cue
- F-actin treadmilling slows
- F-actn accumulates (which stabalises the filopodia and drags tubulin the into the filopodia)
- molecular clutch is engaged with results in forward movement of filopodia
- an actin-tubulin link pulls the microtubules into the extending filopodium
what happens when growth cones are repelled
when growth cones come into contact with non-permissive factors or chemorepellents, they make an attachement then get a signal to go away, and the growth cone collapses due to f-actin destabalisation
non-permissive factors
contact repellents which do not allow growth
permissive facotrs
contact attractions which allow for growth
semaphorins (+3 types)
a family of non-permissive factors that cause collapsing
- Sema1 (membrane) channels axons in grasshopper limb
- Sema2 (secreted) forms gradient that directs axons in grasshopper limb
- Sema3A (secreted) knockout in mice shows axon straying, enhanced by pissoffin (fibulin2)
ephrins
non-permissive factors that bind to Ephs can cause repulsion between cells
early on the compartmentalise embryo into domains, later on they keep axons out of specific areas
laminin
ECM protein localised in optic nerve which is a permissive factor in certain concentration range, but it is not instructive
netrin
secreted by the floorplate and acts as a chemoattractant to turn commissural axons
BMP7
secreted by roofplate and acts as a chemorepellant to cause commissural growth cone collapse
SHH
chemoattraction secreted by floorplate which pulls commissural axons.
- Smo KO and cyclopamine (which blocks SHH signalling) disrupt axon guidance
evidence of how growth cones can be reprogrammed
1) commissural axons lose responsiveness to Netrin after crossing midline and continue without turning (except spinal cord)
2) commissural axons become sensitive to repellants only after crossing the floor plate. repellants in spinal cord create a channel for axons to turn and grow on
inhibitory repellants that axons become sensitive to after crossing midline
semaphorins and slits
Robo
encodes slit receptor on growth cone, protein levels increase after crossing midline so it detects inhibitory repellants
Comm
regulates Robo by encoding trafficking protein which only lets Robo to the cell surface after crossing the midline so that slit can be detected
Robo mutant
cannot detect slit so they keep crossing the midline
Comm mutant
robo levels are high in axons that normally cross but now they dont
vertebrate robo
robo1 - expressed before and after midline
Rig1 (robo3)
expressed before midline and blocks robo1 until midline is crossed
hwo do axons stay and get off of scaffolds
by controlling fasciculation through homophilic binding of cell adhesion molecules (CAMs) like fasciclin II
Fasciclin II
a cell adhesion molecule which controls fasciculation
BEAT
regulates FasII and interferes with adhesion
Fas mutant and Fas overexpression
results in many desfasciculated axons, extra fasiculuation
two types of target selection
1) discrete targets -> axons look for labels on their targets
2) topographic maps -> when neighboring axons go to neighboring sites on target to maintain topology in target
fasciclin 3
adhesion molecule on muscles and axons that acts as label
example of topographic map
posterior tectum expresses gradient of ephrin A2/A5 which repels temporal axons
*non-permissive factors can be used instructively!!!
