Neuroscience lectures 4-5 Axon Growth and Guidance

Question 1

What is the growth cone?

Answer 1

A specialised structure at the tip of extending axons that guides the growing axon to its synaptic target

Question 2

What are the 2 main structures of the growth cone? Briefly explain each

Answer 2

1) Fine extensions called Filopodia
2) A flat mesh of dense actin called Lamellipodia

The structure is much like a webbed hand - Filopodia are the fingers and the webbed skin in the Lamellipodia

Question 3

State the 2 aspects of the growth cone cytoskeleton and where each are found

Answer 3

1) Microtubules
- concentrated in axon shaft and in central domain of growth cone
- Some project forward to leading edge and ino filopodia
- Plus end directed towards leading edge

2) Actin filaments
- Concentrated at leading edge and filopodia
- plus end points forward

Question 4

What are the main roles of microtubules and actin in the growth cone?

Answer 4

Microtubules transport nutrients from cell body to growth cone to support actin elongation

Actin filament dynamics drive directional movement of growth cone

Question 5

What is actin treadmilling?

Answer 5

The process by which the actin filament grows to drive growth cone forward

Question 6

What is the process of actin treadmilling?

Answer 6

1) Actin polymerised at leading edge by addition of G-actin monomers to filament (F-actin)
2) Actin filament is moved backwards by the action of non-muscle myosin (retrograde flow)
3) Depolymerisation occurs in the central region of the growth cone and the G-actin monomers are recycled for assembly at the leading edge

Question 7

What factor is needed for growth cone advance

Answer 7

Mechanical tension/Substrate adhesion

Question 8

How does substrate adhesion allow growth cone advance?

Answer 8

Links actin to the substrate

• Provides traction for growth cone
• As actin is held in place, new monomers are added to the leading edge

Question 9

What 2 factors do movement of the growth cone depend on?

Answer 9

• Its own motile capability

- Interaction with the substrate

Question 10

How do growth cones navigate their way to target cells? (Growth cone guidance)

Answer 10

• By detecting guidance cues (specific molecules) present in the environment of developing neurons through the receptors on the growth cone membrane

Question 11

What are the 2 types of guidance cues?

Answer 11

1) Permissive/Attractive
- positive effect on growth cone movement

2) Repulsive
- negative effect on growth cone movement

Question 12

What are the sources of guidance cues?

Answer 12

1) Extracellular matrix

2) Cells in the path of the growth cone
- cell surface: contact or short-range guidance
- diffusible: chemotrophic or long-range guidance

Question 13

Name some characteristics of guidance cue families

Answer 13

• Individual families may contain both diffusible and membrane associated cues
• A given molecule may acts as an attractant or a repellent depending on the receptor expressed

Question 14

What are Netrins?

Answer 14

Small family of secreted guidance cues (3 in mammals)

Question 15

What are the receptors for Netrins?

Answer 15

• Deleted in colorectal cancer (DCC)
  > self associates via its intracellular domain to mediate growth cone attraction
• UNC-5
• interacts with DCC

Question 16

How do DCC and UNC-5 respond to netrins?

Answer 16

• DCC alone orientates towards netrin

- UNC-5 on its own or with DCC mediates repulsion

Question 17

What are semaphorins (semas)?

Answer 17

Guidance cues
approx 20 in vertebrates
- divided into 5 classes: class 3-7
- Can act as both attractants and repressors

Question 18

What is the main receptor for semaphorins?

Answer 18

Plexin

Question 19

What is responsible for the interaction between sema and plexin?

Answer 19

Both have a 500 AA extracellular sema domain

Question 20

What type of molecules are slits?

Answer 20

Large secreted proteins encoded by 3 genes in mammals

Question 21

What type of receptors do slits bind to?

Answer 21

Roundabout receptors

Question 22

What type of signal do slits give?

Answer 22

• Repulsive

- Also involved in regulating axonal and dendritic branching

Question 23

What are the 2 classes of ephrins?

Answer 23

Class A and Class B

Question 24

How many class A and B ephrins are there and how are they attached to the cell?

Answer 24

Class A:
5 in humans
Attached to cell membrane via GPI anchor

Class B:
3 in humans
Single pass transmembrane proteins

Question 25

What are the 2 types of ephrin receptors? What molecule are they?

Answer 25

EphA (9 in humans) bind class A
EphB (5 in humans) bind class B

Tyrosine kinase receptors

Question 26

What type of signalling in Ephrin-Eph signalling?

Answer 26

• Usually short range
• Class A ephrins can sometimes be released to signal at distance
• may be repulsive or attractive

Question 27

Where do motorneurons differentiate and under what controls?

Answer 27

Differentiate in the ventral horns of spinal cord under control of sonic hedgehog which induces homeobox gene expression

Question 28

Where do axons initially extend over?

Answer 28

Block of mesodermal tissue called somites outside of the spinal cord

Question 29

What is a sclerotome?

Answer 29

Part of the somite

• gives rise to cartilage and bone of vertebral column and ribs
• has anterior and posterior parts

Question 30

How does ephrin-eph signalling control motorneuron growth cone guidance?

Answer 30

Motor neuron growth cone avoids the ephrin-expressing posterior sclerotome and choose the anterior half that lacks ephrin

Question 31

What assay is used to characterise the repulsive guidance cues

Answer 31

Stripe assay

Question 32

What is within each lateral motor column?

Answer 32

Motor pools that innervate different muscle types

Question 33

What muscles to lateral and medial LMCs innervate

Answer 33

Lateral LMCs - dorsal limb muscles

Medial LMCs - ventral limb muscles

Question 34

How does repulsive ephrin signalling controll growth of these columns to the correct muscle type?

Answer 34

LMC-lateral axons = dorsal
LMC-medial axons = ventral

Doral limb expresses ephrin B
Ventral limb expresses ephrin A

LMC-L growth cones express EphA receptor
LMC-M growth cones express EphB receptor

Repulsion means that grow in opposite parts

Question 35

How is dorsal to ventral guidance supplied by long range chemoattraction

Answer 35

• Netrin secreted for floor plate cells - attracts growth cones expressing DCC receptor
• sHg and VEGF also act as attractants
• Combined with ‘push from behind’
• BMP produced from roof plate - acts as repellent

Question 36

How are growth cones navigated to the midline?

Answer 36

Chemoattraction followed by repulsion:

• Netrin attracts axon across midline
• Slit secreted by floor plate as chemo repellent
• Axons initially insensitive to slit - crossing upregulates Robo receptors i.e repulsion switched on after crossing
• Crossing switches off attraction

A balance between attraction and repulsion gets axon to midline

Question 37

How is ventral to dorsal growth cone guidance achieved?

Answer 37

Axons with frizzled receptor for Wnt grow up a Wnt concentration gradient

Question 38

What is Horizontal Gaze Palsy with Progressive Scoliosis (HGPPS)

Answer 38

• Rare autosomal recessive disorder
• Mutations in Robo 3
• Abnormal axon crossing of midline
• Inability to coordinate lateral (left-right) eye movements (can’t look left or right)
• Postural and motor problems
• Vertical eye movement normal

Question 39

What is Congential Mirror Movements?

Answer 39

Autosomal dominant disorder

• Mutations in DCC gene
• Voluntary movements on one side of body mirrored simultaneously in opposite side