Neuroscience lectures 4-5 Axon Growth and Guidance Flashcards
What is the growth cone?
A specialised structure at the tip of extending axons that guides the growing axon to its synaptic target
What are the 2 main structures of the growth cone? Briefly explain each
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
State the 2 aspects of the growth cone cytoskeleton and where each are found
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
What are the main roles of microtubules and actin in the growth cone?
Microtubules transport nutrients from cell body to growth cone to support actin elongation
Actin filament dynamics drive directional movement of growth cone
What is actin treadmilling?
The process by which the actin filament grows to drive growth cone forward
What is the process of actin treadmilling?
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
What factor is needed for growth cone advance
Mechanical tension/Substrate adhesion
How does substrate adhesion allow growth cone advance?
Links actin to the substrate
- Provides traction for growth cone
- As actin is held in place, new monomers are added to the leading edge
What 2 factors do movement of the growth cone depend on?
- Its own motile capability
- Interaction with the substrate
How do growth cones navigate their way to target cells? (Growth cone guidance)
- By detecting guidance cues (specific molecules) present in the environment of developing neurons through the receptors on the growth cone membrane
What are the 2 types of guidance cues?
1) Permissive/Attractive
- positive effect on growth cone movement
2) Repulsive
- negative effect on growth cone movement
What are the sources of guidance cues?
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
Name some characteristics of guidance cue families
- 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
What are Netrins?
Small family of secreted guidance cues (3 in mammals)
What are the receptors for Netrins?
- Deleted in colorectal cancer (DCC)
> self associates via its intracellular domain to mediate growth cone attraction - UNC-5
- interacts with DCC
How do DCC and UNC-5 respond to netrins?
- DCC alone orientates towards netrin
- UNC-5 on its own or with DCC mediates repulsion
What are semaphorins (semas)?
Guidance cues
approx 20 in vertebrates
- divided into 5 classes: class 3-7
- Can act as both attractants and repressors
What is the main receptor for semaphorins?
Plexin
What is responsible for the interaction between sema and plexin?
Both have a 500 AA extracellular sema domain
What type of molecules are slits?
Large secreted proteins encoded by 3 genes in mammals
What type of receptors do slits bind to?
Roundabout receptors
What type of signal do slits give?
- Repulsive
- Also involved in regulating axonal and dendritic branching
What are the 2 classes of ephrins?
Class A and Class B
How many class A and B ephrins are there and how are they attached to the cell?
Class A:
5 in humans
Attached to cell membrane via GPI anchor
Class B:
3 in humans
Single pass transmembrane proteins
What are the 2 types of ephrin receptors? What molecule are they?
EphA (9 in humans) bind class A EphB (5 in humans) bind class B
Tyrosine kinase receptors
What type of signalling in Ephrin-Eph signalling?
- Usually short range
- Class A ephrins can sometimes be released to signal at distance
- may be repulsive or attractive
Where do motorneurons differentiate and under what controls?
Differentiate in the ventral horns of spinal cord under control of sonic hedgehog which induces homeobox gene expression
Where do axons initially extend over?
Block of mesodermal tissue called somites outside of the spinal cord
What is a sclerotome?
Part of the somite
- gives rise to cartilage and bone of vertebral column and ribs
- has anterior and posterior parts
How does ephrin-eph signalling control motorneuron growth cone guidance?
Motor neuron growth cone avoids the ephrin-expressing posterior sclerotome and choose the anterior half that lacks ephrin
What assay is used to characterise the repulsive guidance cues
Stripe assay
What is within each lateral motor column?
Motor pools that innervate different muscle types
What muscles to lateral and medial LMCs innervate
Lateral LMCs - dorsal limb muscles
Medial LMCs - ventral limb muscles
How does repulsive ephrin signalling controll growth of these columns to the correct muscle type?
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
How is dorsal to ventral guidance supplied by long range chemoattraction
- 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
How are growth cones navigated to the midline?
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
How is ventral to dorsal growth cone guidance achieved?
Axons with frizzled receptor for Wnt grow up a Wnt concentration gradient
What is Horizontal Gaze Palsy with Progressive Scoliosis (HGPPS)
- 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
What is Congential Mirror Movements?
Autosomal dominant disorder
- Mutations in DCC gene
- Voluntary movements on one side of body mirrored simultaneously in opposite side
