axon guidance Flashcards
how many synaptic connections are there in the human brain?
10^14
resonance theory (proposed by Weiss)
early, random and diffuse outgrowth to all targets
then elimination of non-functional connections
chemoaffinity hypothesis (proposed by Sperry)
directed + specific outgrowth of axons by following individual identification tags carried by the environment
experiment showing chemoaffinity hypothesis is correct
cut optic nerve and remove temporal retina, allowing just the nasal axons to extend.
RESULT: nasal axons extends only go to the posterior tectum (where they should go)
IF WEISS WAS CORRECT: nasal axons will grow everywhere and then non-functional axons will be eliminated
experiment showing motor axons can navigate to their targets
control = cut and replace segment of neural tube (T7-LS3) before motor axons grow out —> normal innervation of specific limb muscles by motor axons from T7-LS1
cut and reverse T7-LS3 before motor axons grow out —> motor axons still find their way to their normal muscle targets
DEFINE: growth cone
growing tip of the axon which senses cues in the environment
DEFINE: pioneers
early axons
form an axon scaffold on which followers can extend
DEFINE: followers
later axons
extend on an axon scaffold formed by pioneers
Ti1
pioneer axon
describe the pathway of pioneer axons in the grasshopper embryo limb
Ti1 extends, makes a turn at the limb/body boundary and then turns again to innervate cx1 cell
limb/body boundary contains an inhibitory cue
cx1 cell contains an attractive cue
DEFINE: guidepost cells/stepping stones
cells in the pathway of pioneer axons that cause stalling when ablated
what are the 4 different ways guidance cues act in?
- contact attraction
- contact repulsion
- chemoattraction
- chemorepulsion
DEFINE: contact attraction
attract when in contact
DEFINE: contact repulsion
repel when in contact
DEFINE: chemoattraction
attracted to chemicals
DEFINE: chemorepulsion
repelled by chemicals
experiment showing the labelled pathway hypothesis is true
g axon extends horizontally across midline and makes a turn
ablate p axon –> g axon stalls. other axons still present so not due to lack of axons on which to extend
ablate a axon –> g axon unaffected so not due to reduced number of axons
so p axon carries a guidance cue sensed by g axon growth cone
why are aplysia growth cones studied?
they are flat so they can be observed easily
how is f actin crosslinked in lamella
actin bundles crosslinked into a net
how is f actin crosslinked in filopodia
actin bundles forming larger bundles
growth cone when in contact with attractive cue??
- f actin treadmilling slows
- f actin accumulation near attractive cue
- microtubules reorganised - establishes new growth direction
describe how the cytoskeleton is rearranged during filopodial extension
- growth cue binds to receptor on filopodium
- actin filament crosslinking
- actin treadmilling directed towards back slows
- actin-tubulin pulls microtubules forward in direction of extending filopodium
- forward movement of filopodium towards attractive cue = filopodial extension
growth cone when in contact with attractive cue???
- growth cone collapse
- f actin destabilised –> lower F-actin levels in growth cone
- actin initiates new growth cones going in a new direction
what are semaphorins?
inhibitory guidance cues
what are permissive substrates?
contact attractants
how are ephrins used during early patterning?
ephrins are non-permissive, contact repulsion ligands
cause repulsion between cells
compartmentalises embryo into discrete domains e.g. rhombomeres
what is cre recombinase encoded by?
bacteriophage p1
what are netrins?
chemoattractant secreted by floor plate and muscles
expressed along midline of NS
diffuses through ECM (collagen)
attracts axons towards it
causes commissure axons to turn towards floor plate
enables motor neurons to innervate their muscle targets
what is reprogramming growth cones?
changing the sensitivity of a growth cone to specific cues as intermediate targets are encountered
lipophilic dyes are fluorescent and are absorbed into the membrane to highlight the membrane. what are their possible uses?
- membrane tracking
- lineage tracing
- axon tracing
what are slits?
cell surface repellants expressed in the floor plate
what is Robo?
receptor for slit
what would be observed in a Comm mutant?
- axons do not cross the midline –> no commissures
- increased robo protein levels in axons normally crossing midline
what is Comm?
trafficking protein preventing robo protein from reaching the cell surface and responding to slits
what is Rig1/Robo3?
inhibits robo1 signalling in response to slits
expressed only in pre-crossing fibres
what would be observed in a Rig1/Robo3 mutant?
commissure axons do not cross midline (premature repulsion to slits)
what are BEAT proteins?
promote defasciculation by disrupting CAM-mediated adhesion e.g. via fascicilin II
what is fascicilin 3?
homophilic cell adhesion molecule
which cues are involved in attracting growth cones to specific targets?
- netrin
- fascicilin 3
co-ordinate system for achieving topographic maps
encoded by gradients of signalling molecules expressed by the target read by complementary gradients of receptors expressed on axons
TRUE OR FALSE: ephrin A2 and A5 are not instructive
false
describe the stripe assay used to prove the co-ordinate system theory of topographic maps
- isolated anterior and posterior membranes of the tectum and put them down in alternating stripes in culture
- strip of retina placed down the side of alternating stripes
- nasal axons extend anteriorly or posteriorly
temporal axons only extend anteriorly
gradient of ephrin a2 and a5 expressed in tectum from posterior to anterior
counter gradient of eph a3 receptors expressed in retinal ganglion cells from temporal to nasal
