lec 11-13. axon guidance

Question 1

Weiss resonance theory

Answer 1

random neuronal outgrowth to all targets followed by elimination of non-functional connections

Question 2

Sperry chemoaffinity hypothesis

Answer 2

axons grow directly and specifically to individual ID tags carried by cells of the embryo

Question 3

how sperry tested his hypothesis

Answer 3

cut the optic nerve and removed the temporal retina. nasal neurons grew specifically to the posterior end -> sperrys theory was right

Question 4

why insects are used for axon guidance experiments

Answer 4

• have simple nervous systems
• embryos are easy to observe and manipulate
• can ablate larger cells like in grasshopper

Question 5

labelled pathway hypothesis

Answer 5

axons express different receptors on their growth cones and can selectively fasciculate with other axons that carry labels.

Question 6

axon scaffolds

Answer 6

pioneers form an axon scaffold that followers can extend on (which are important in vertebrates too!)

Question 7

example of axon scaffolds in vertebrates

Answer 7

subplate neurons which project into the thalamus provide a scaffold for LGN neurons to reach the cortex

Question 8

grasshopper embryo limb experiment

Answer 8

• 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

Question 9

four types of guidance cues

Answer 9

chemoattraction
chemorepulsion
contact attraction
contact repulsion

Question 10

how does a neuron know which end the growth cone should be

Answer 10

axons express Tau, and dendrites express MAP2, giving the neuron polarity, which is determined by neurite selection

Question 11

neurite selection

Answer 11

appears to be random after different neurites are ‘tried out’.
microtubule stabalisation is critical for axon formation

Question 12

new axons have ____ microtubules

polarised axons have ____ microtubules

Answer 12

new axons - tyrosinated dynamic microtubules

polarised axons - acetylated stabalised microtubules

Question 13

lamellopodia

Answer 13

made up of F-actin. actin bundles are cross-linked into net

Question 14

filopodium

Answer 14

made up of F-actin. acting bundles are polarised to form larger bundles

Question 15

resting growth cone movement

Answer 15

F-actin treadmills and tubulin sporadically gets dragged into filopodium

Question 16

growth cone movement when in contact with attractive cue

Answer 16

• 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

Question 17

what happens when growth cones are repelled

Answer 17

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

Question 18

non-permissive factors

Answer 18

contact repellents which do not allow growth

Question 19

permissive facotrs

Answer 19

contact attractions which allow for growth

Question 20

semaphorins (+3 types)

Answer 20

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)

Question 21

ephrins

Answer 21

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

Question 22

laminin

Answer 22

ECM protein localised in optic nerve which is a permissive factor in certain concentration range, but it is not instructive

Question 23

netrin

Answer 23

secreted by the floorplate and acts as a chemoattractant to turn commissural axons

Question 24

BMP7

Answer 24

secreted by roofplate and acts as a chemorepellant to cause commissural growth cone collapse

Question 25

SHH

Answer 25

chemoattraction secreted by floorplate which pulls commissural axons.
- Smo KO and cyclopamine (which blocks SHH signalling) disrupt axon guidance

Question 26

evidence of how growth cones can be reprogrammed

Answer 26

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

Question 27

inhibitory repellants that axons become sensitive to after crossing midline

Answer 27

semaphorins and slits

Question 28

Robo

Answer 28

encodes slit receptor on growth cone, protein levels increase after crossing midline so it detects inhibitory repellants

Question 29

Comm

Answer 29

regulates Robo by encoding trafficking protein which only lets Robo to the cell surface after crossing the midline so that slit can be detected

Question 30

Robo mutant

Answer 30

cannot detect slit so they keep crossing the midline

Question 31

Comm mutant

Answer 31

robo levels are high in axons that normally cross but now they dont

Question 32

vertebrate robo

Answer 32

robo1 - expressed before and after midline

Question 33

Rig1 (robo3)

Answer 33

expressed before midline and blocks robo1 until midline is crossed

Question 34

hwo do axons stay and get off of scaffolds

Answer 34

by controlling fasciculation through homophilic binding of cell adhesion molecules (CAMs) like fasciclin II

Question 35

Fasciclin II

Answer 35

a cell adhesion molecule which controls fasciculation

Question 36

BEAT

Answer 36

regulates FasII and interferes with adhesion

Question 37

Fas mutant and Fas overexpression

Answer 37

results in many desfasciculated axons, extra fasiculuation

Question 38

two types of target selection

Answer 38

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

Question 39

fasciclin 3

Answer 39

adhesion molecule on muscles and axons that acts as label

Question 40

example of topographic map

Answer 40

posterior tectum expresses gradient of ephrin A2/A5 which repels temporal axons

*non-permissive factors can be used instructively!!!