Lecture 12 Axon guidance 2 Flashcards
How are neurites selected in vitro
MT stability
Competition between neurites
Feedback loop
When is neuron polarity established in vivo?
+ example
As neurons are born
Suggests neurite selection is based and influenced by the cellular environment
e.g. radial glia already polarised apical to basal and polarity preserved as cell divide
Growth cones of what organism are easier to image?
Aplysia
GC sits down more
What are the three domains of growth cones?
C DOMAIN - central
T ZONE - transitional
P DOMAIN - peripheral
What are made from different types of F-actin?
filopodia and lammelipodia
What are filopodia?
Type of protrusion
- Finger-like projections
- Core of long, bundled actin filaments
What are lamellipodia?
Type of protrusion
- Sheet-like structures
- Crosslinks into a net
What is the action of F-actin in growth cones?
Treadmills in resting growth cone
Describe actin treadmilling
Tubulin is dragged sporadically into Filopedia
Flow from periphery to centre
Filaments break up and recycled to tip
What happens to F-actin treadmilling when the cell experiences attractive cue?
Tubulin are dragged more dramatically to filopedia
How do growth cones change direction?
They do not turn They reorganise their MT 1. GC contacts attractive cue 2. F actin treadmilling slows 3. F actin accumulates 4. This stabilises filopodium and drags MT to back of filopodium
Is a substrate sufficient enough to drive forward movement?
NO
What else is needed to drive movement forward?
Stimulus of cue to rearrange cytoskeleton
What part of filopodium is attached to substrate
C domain = palm
Discuss actin treadmilling process before and after attractive cue encountered
- slow undirected growth - resting - max actin flow
- Growth cue encountered - filopedia extension and reorientation of MT - fast directed growth. Accumulation of F actin to stabilise filopodium and drag MT to back of filopodium
- MOLECULAR CLUTCH - slows rearward (back) treadmilling
- ACTIN-TUBULIN LINK - pulls microtubules into the wake of extending filopodium (i.e. back into filopodium)
What can cues do to growth cones? + experiment
attract and repel them
e.g. mix RGC and SN - fasiculate only with own type due to repulsion
What happens when growth cones encounter repelling cue?
Growth cone collapse - STILL ATTACH - retreat and reorganise
F actin is destabilised and get decrease in F actin (opposite to attractive cue)
Example of family of inhibitory cues? Found by?
Semaphorins
Biochem pur of retina showing collapse of sensory axons
Semaphorin types
- membrane bound
2. secreted
Example secreted semaphorin
Sema 3A
What do semaphorins cause?
Collapse of F actin
What is essential for axon growth? How was this shown
axon attachment via palm
Embryonic chick DRG neurons cell bodies and axons - Polyornithine sticky, palladium non-sticky
Relationships between adhesion and outgrowth
NO RELATIONSHIP
What ECM component does DRG stick better to (adhesion)
Collagen
What ECM component is DRG growth better on
Laminin
What can permissive factors do? Example
Can define substrate paths in the embryo
Laminin allows RGC to enter tectum - promoting ECM in optic nerve
Other name for permissive factors
Contact attractants
What do permissive factors not dictate? Shown by
direction - only allow growth
- block laminin R –> decrease in retinal growth but direction unchanged
- Gradients of laminin in vitro don’t alter growth (if in certain range)
Permissive factors are x2
Permissive to allow growth
Not instructive to alter direction
What are non-permissive factors often called?
Contact repellents
What can non-permissive factors do?
Channel axon growth
What does outgrowth depend on in presence of non-permissive factors
Permissive factors
Example of non-permissive factor and its function
- semaphorins
- tell an axon it can’t grow there
e. g. Grasshopper limb bud Ti1 –> Cx1 turns at semaphoring zones
What occurs in mice lacking Sema 3A
End in wrong territories
Therefore, what is axon growth a balance between?
permissive and non-permissive factors
What are ephrins?
non-permissive factors used in early patterning and to guide axons
What are ephs?
cell surface molecules detected by receptors
What do ephs and ephrins cause?
REPULSION between cell
What are the ephs and ephrins important for?
- compartmentalisation in early development into distinct domains e.g. rhombomeres
- late dev for keeping axon out of specific areas as do semaphorins
- making topographic maps - reciprocal patterning
Where are ephs restricted to in vertebrate
Proximal distal limb tips
Where are ephrins restricted to in vertebrate
Middle
What does the floor plate secrete that effects axon growth?
Netrin
What do we know key patterning organisers secrete?
ong distance guiding molecules - chemoattractant and chemorepellents
These guide the axons (where as short distant just say where can and cant go)
What does netrin do?
act as a chemoattractant for axons - makes axons grow towards it
Where is netrin expressed
Vert midline of NS
What do floor plate cells and cells expressing netrin gene turn on
Commissural axons
What does the roof plate secrete that affects axon growth?
a chemorepellent in the form of a BMP - doesn’t allow axons to gorw in that direction
What does BMP7 cause
Growth cone collapse
A combination of what causes axon growth towards floor plate?
Push factor = BMP
Pull factor = Netrin
Example of long and short cues working together
Sema 1 cells surface - blocking with AB means axon stays in wrong are
Sema 2 secreted - disrupts Ti guidance
