L11 - Axon Guidance II

Question 1

What are the 3 growth cone domains?

Answer 1

Central
Transitional
Peripheral

Question 2

What is lamella made of?

Answer 2

F-actin bundles crosslinked into a net

Question 3

What is filopodia made off?

Answer 3

F-actin bundles polarised to form larger bundles

Question 4

How does F-actin treadmill in a resting growth cone?

Answer 4

Tubulin is dragged sporadically into the filopodia

Question 5

What happens when a growth cone comes in contact with an attractive cue?

Answer 5

F-actin treadmilling slows
F-actin accumulates
Stabilises the filopodium
Drags microtubules into back of filopodium

If bead immobile, growth cone would reorganise its microtubules establishing a new growth direction

Question 6

When a growth promoting cue is encountered what two things lead to filopodial extension and reorientation of microtubules?

Answer 6

Molecular Clutch is engaged and rearward actin treadmilling slows
- Results in forward movement of filopodium
Actomyosin-based actin-tubulin link pulls microtubules into wake of extending filopodium

Question 7

Attachment of growth cone to substrate is not enough to drive forward movement you also need?

Answer 7

Stimulus of attractive cue

Question 8

What are semaphorins?

Answer 8

Family of inhibitory guidance cues

Question 9

How were semaphorins identified?

Answer 9

Biochemical purification of factor from retina responsible for the collapse of sensory axons

Question 10

What are the two types of semaphorins?

Answer 10

Membrane-bound

Secreted (e.g. Sema3A)

Question 11

How do semaphorins cause growth cones to turn?

Answer 11

They have a collapsing effect primarily on F-actin

Question 12

Why do growth cones need substrates which are permissive for growth?

Answer 12

Attachment alone is not enough

Question 13

Is there a relationship between strength of adhesion and amount of axon growth?

Answer 13

No

Question 14

Outgrowth on different extra-cellular matric components

Answer 14

Adhesion: laminin < collagen
Outgrowth: collagen &laquo_space;laminin

Question 15

What is laminin?

Answer 15

Growth-promoting extracellular matrix protein, is localised in the optic nerve
Does not dictate direction of axon growth

Question 16

Blockade of receptors for laminin shows?

Answer 16

Slowing down of growth of retinal axons, but does not change direction

Question 17

Gradients of laminin cause?

Answer 17

Do not direct axon growth
Permissive for growth within a specific concentration range
- Permissive but not instructive

Question 18

What do permissive substrates do?

Answer 18

Contact attractants

Help axon growth

Question 19

What do non-permissive substrates do?

Answer 19

Contact repellants
Not a lack of adhesion
Help channel axon growth

Question 20

Is there a relationship between adhesiveness and permissiveness?

Answer 20

No

Question 21

Mice lacking Sema 3A see?

Answer 21

Axons straying into wrong territories

Question 22

What are Ephrins?

Answer 22

Non permissive factors used in early patterning and to guide axons

Question 23

What do Ephrins do?

Answer 23

Cause repulsion between cells

• Early on - help compartmentalise embryo into discrete domains e.g. rhombomeres
• Later on - keep axons out of specific areas

Question 24

Ephrin patterning

Answer 24

They have a reciprocal pattern of expression in the mammalian embryo

Question 25

What are chemoattractants and chemorepellents?

Answer 25

Key patterning organisers secrete these long distance guiding molecules

Question 26

What led to cloning of the gene encoding the floor plate chemoattractant protein?

Answer 26

Biochemical purification

• Expressed along midline of vertebrate nervous system
• Protein named Netrin

Question 27

What is Netrin?

Answer 27

Secreted protein which can associate with extracellular matrix
Chemoattractant protein

Question 28

Commissural axons are repelled by?

Answer 28

BMPs made by the roof plate

Question 29

What is the role of BMPs?

Answer 29

Determine which neurons are specified in spinal cord
Play a role in patterning axon pathways
- Purified BMP7 causes commissural growth cone collapse
- Cells expressing BMP7 mimic repulsion of the roof plate

Question 30

Some commissural axons reach the floor plate without netrin

Answer 30

Cyclopamine blocks SHH signalling

Smo is required for SHH signalling

Question 31

What is the tole of Cre recombinase and loxP?

Answer 31

Used to delete DNA lying between two loxP sites
Bacteriophage P1 encodes Cre that enables it to insert its DNA into host bacteria’s genome
Cre binds to a 34 base pair sequence, loxP, which it can cut and rejoin to another loxP site

Question 32

What is a floxed gene?

Answer 32

A gene flanked by loxP sites

Question 33

Gradients of morphogens are reused to shape axon paths

Answer 33

Early patterning information is used to guide pioneer axons
Chemoattractant and chemorepulsive molecules work together to guide commissural axons to their initial target - floor plate

Question 34

What is the role of Sema 1?

Answer 34

Short-range cue

Blocking its function with antibodies leads to axons in wrong areas

Question 35

What is the role of Sema2?

Answer 35

Blocking its functions disrupts Ti1 guidance

Suggests gradient of Sema2 directs Ti1 growth cone towards body