L11 - Axon Guidance II Flashcards
What are the 3 growth cone domains?
Central
Transitional
Peripheral
What is lamella made of?
F-actin bundles crosslinked into a net
What is filopodia made off?
F-actin bundles polarised to form larger bundles
How does F-actin treadmill in a resting growth cone?
Tubulin is dragged sporadically into the filopodia
What happens when a growth cone comes in contact with an attractive cue?
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
When a growth promoting cue is encountered what two things lead to filopodial extension and reorientation of microtubules?
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
Attachment of growth cone to substrate is not enough to drive forward movement you also need?
Stimulus of attractive cue
What are semaphorins?
Family of inhibitory guidance cues
How were semaphorins identified?
Biochemical purification of factor from retina responsible for the collapse of sensory axons
What are the two types of semaphorins?
Membrane-bound
Secreted (e.g. Sema3A)
How do semaphorins cause growth cones to turn?
They have a collapsing effect primarily on F-actin
Why do growth cones need substrates which are permissive for growth?
Attachment alone is not enough
Is there a relationship between strength of adhesion and amount of axon growth?
No
Outgrowth on different extra-cellular matric components
Adhesion: laminin < collagen
Outgrowth: collagen «_space;laminin
What is laminin?
Growth-promoting extracellular matrix protein, is localised in the optic nerve
Does not dictate direction of axon growth
Blockade of receptors for laminin shows?
Slowing down of growth of retinal axons, but does not change direction
Gradients of laminin cause?
Do not direct axon growth
Permissive for growth within a specific concentration range
- Permissive but not instructive
What do permissive substrates do?
Contact attractants
Help axon growth
What do non-permissive substrates do?
Contact repellants
Not a lack of adhesion
Help channel axon growth
Is there a relationship between adhesiveness and permissiveness?
No
Mice lacking Sema 3A see?
Axons straying into wrong territories
What are Ephrins?
Non permissive factors used in early patterning and to guide axons
What do Ephrins do?
Cause repulsion between cells
- Early on - help compartmentalise embryo into discrete domains e.g. rhombomeres
- Later on - keep axons out of specific areas
Ephrin patterning
They have a reciprocal pattern of expression in the mammalian embryo
What are chemoattractants and chemorepellents?
Key patterning organisers secrete these long distance guiding molecules
What led to cloning of the gene encoding the floor plate chemoattractant protein?
Biochemical purification
- Expressed along midline of vertebrate nervous system
- Protein named Netrin
What is Netrin?
Secreted protein which can associate with extracellular matrix
Chemoattractant protein
Commissural axons are repelled by?
BMPs made by the roof plate
What is the role of BMPs?
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
Some commissural axons reach the floor plate without netrin
Cyclopamine blocks SHH signalling
Smo is required for SHH signalling
What is the tole of Cre recombinase and loxP?
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
What is a floxed gene?
A gene flanked by loxP sites
Gradients of morphogens are reused to shape axon paths
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
What is the role of Sema 1?
Short-range cue
Blocking its function with antibodies leads to axons in wrong areas
What is the role of Sema2?
Blocking its functions disrupts Ti1 guidance
Suggests gradient of Sema2 directs Ti1 growth cone towards body
