How neurons develop their shapes Flashcards
How are axons and dendrites FUNCTIONALLY distinct?
Dendrites:
- Collect and integrate incoming information
Axons:
- Carry information away from the cell body in action potentials
How are axons and dendrites STRUCTURALLY distinct?
Axons:
- Highly polarised microtubules
Dendrites:
- Microtubules are LESS ORDERED and have MIXED ORIENTATION
How are axons and dendrites MOLECULARLY distinct?
Have DIFFERENT MAPs
Axons: Tau
Dendrites: MAP2
These MAPs cross-link MTs DIFFERENTLY
What are MAPs and why are they needed
Microtubule assembly proteins needed to stabilise microtubules
How do MAP stabilise microtubules?
Cross-link them into bundles
What are the MAPs in axons?
Tau
What are the MAP in dendrites?
MAP2
How is the plasma membrane organised?
COMPARTMENTALISED both intracellularly and on the cell membrane
What is L1?
An adhesion molecule that is restricted to AXONS
What is GluR?
Glutamate receptor that is restricted to the CELL BODY and DENDRITES
How is L1 restricted to the axon?
It is inserted at the GROWTH CONE as the growth cone grows
What is the experiment to show how surface separation is maintained?
What were the results?
What is the conclusion?
- Bead coated with antibodies for L1 or GluR1
- Drop bead onto dish - sticks via antibody to the component interested in
- Use optical tweezers (laser) to move the bead around in the dish
- Measure the force involved in moving the bead
Results:
- Move L1 beads easily in axonal domain
- L1 beads cannot cross the axon hillock
- Cannot move Glut1 into the axon
Conclusion:
- PHYSICAL DIVISION between the dendritic and the axonal domain
How is the physical division between the dendritic and the axonal domain formed?
Due to interaction between the CELL SURFACE molecules and the UNDERLYING CYTOSKELETON
What is the experiment that shows how neuron polarity is set up?
What was seen?
Dissected hippocampal neurons dissociated from early mouse embryo and watched them develop
Seen:
- Initial sprouts and immature neurites
- Ultimately one neurite becomes DOMINANT - growth cone extends away
- Neurites not selected become DENDRITES
What is a neurite?
ANY projection away from the cell body
What molecules are used to mark where axons are forming?
GFP-labelled + end directed kinesins (Kif-1)
What is critical for axon initiation?
How does this occur?
Microtubule stabilisation:
- Growth cones and neurites contain DYNAMIC/UNSTABLE microtubules
- Newly polarised axon contains STABILISED microtubules
What is the difference between dynamic and stabilised microtubules?
Dynamic microtubules are TYROSINATED
Stabilised microtubules are ACETYLATED
How can an axon be artificially chosen?
What does this suggest?
Artificially stabilise MTs using TAXOL treatment of one neurite
Suggests:
- Competition between neurites to stabilise their MTs
- Some come of kind of feedback loop to SUPPRESS other neurites from becoming axons once a decision has been made
What happens when cut oft the growth cone of a neuron in culture?
What does this show?
Another neurite becomes the growth cone
Shows there is a FEEDBACK LOOP to supress other neurites from becoming axons
What is the feedback loop involved in the choice of a neurons?
BOTH positive and negative:
Positive - reinforce decision to become an axon at the growth cone
Negative - to the rest of the cell
What are the two theoretical models of feedback that prevents other neurites from becoming a neuron (negative feedback)?
What is common in both models?
A) Diffusible molecule released into the cell
B) Something LIMITING in the cell that is needed to form the axon - taken into the chosen neurite and is in LIMITED supply (cannot be used elsewhere)
In common: requirement for a positive feedback to AMPLIFY signal for axon formation
What does over expression of HRas cause?
What does this suggest?
Results in multiple neurons
Suggests that Hras is involved in neurite selection
What happens to HRas as the growth cone forms?
What is this consistent with?
HRas accumulates in the growth cone and is depleted from the rest of the cell (shipped down the axon)
Consistent with the idea of a limited component
What does activating PI3K do to HRas?
What happens to PI3K when HRas is activated?
ACTIVATES HRas
When HRas is activated - upregulate PI3K (feedback loop)
What blocks HRas accumulating in the growth cone as it forms?
What does this show?
PI3K inhibitor blocks HRas accumulation in the growth cone
Shows HRas stabilisation in the growth cone is dependant on PI3K
What happens downstream of PI3K?
PI3K elevates PIP3 levels in the membrane
PIP3 phosphorylated GSKbeta and Akt
Where is PIP3 present?
In the GROWH CONE (not in the immature neurites)
What does PIP3 affect? How?
Affects microtubules via PIP3 effects on Rac which act on actin
Through inhibiting GSKbeta
How does PIP3 inhibit GSK3beta?
In 2 ways:
- DIRECTLY through phosphorylation
- Via PHOSPHORYLATION of Akt, which is activated by PIP3
What does GSK3beta normally do?
Normally INHIBITS microtubule stabilisation - keeping the MT in a dynamic state
How does GSK3beta regulate microtubule stability?
By differentially modulating microtubule association protein activities
What happens to the microtubules when GSK3beta is inhibited?
Inhibition of stabilisation is inhibited
–> stabilisation of the microtubules
How does PIPK effect the Par3 complex?
It PROMOTES Par3 complex accumulation in the growth cone
What is the Par3 complex involved in?
Involved in asymmetric cell division
Where was the Par3 complex discovered?
In c.elegans
What happens if inhibit PIP3 accumulation?
Prevent Par3 accumulation in the growth cone - restricted to the cell body
PREVENTS AXON FORMATION
What inhibits PIP3 accumulation?
PTEN
What are SAD kinases?
Par1-related proteins
What happens when KO SAD kinases?
Why does this happen?
Everything become NEURITES and axons do not form
Faliure of acetylated tubulin (stablised) to predominate over tyrosinated tubulin (dynamic)
What are SAD kinases important in?
Specifying axons
What is LKB1 kinase?
What happens if KO
Par4-related protein
Faliure to make axons
What happens in Par-titioning defective genes?
Mutants fail to develop asymmetric cell divisions
How does the Par complex initiate polarisation?
Through mutually antagonistic interactions of the Par3 complex with the Par1/2 complex
This affects microtubule organisation downstream
Describe the interaction between PI3K, Par3 complex, GSKbeta ,LKB1 and SAD kinase to lead to axon specification
PI3K phosphorylates Par3 complex
Par3 complex INHIBTS GSK3beta (which normally inhibits axon formation) –> axons form
What does LKB1 normally do?
Normally INHIBITS the Par3 complex, meaning GSK3b can function to inhibit axon formation
What does phosphorylation of LKB1 cause?
Inhibits LKB1, inhibiting the inhibition of the Par3 complex, Par3 inhibits GSK3b –> axons can form
ALSO:
ACTIVATES SAD kinase
What is SAD kinase required for?
Axon specification
How does a neuron become polarised?
Inherits polarisation from the division, either:
- Inherited from the cell born from
OR
- From information from the apical basal environment they are in
What phosphorylates LKB1?
PKA
Where is LKB1 found?
Where is phosphorylated LKB1 found?
What is this consistent with?
LKB1: Found EVERYWHERE in the neuron
Phosphorylation LKB1: Restricted to the AXONAL compartment
Consistent with the fact that LKB1 is involved in POLARISATION
What happens if KO LBK1?
No axonal polarisation
Do factors promoting axonal polarity in vitro have effect on axon initiation when they are KO?
No
What medical conditions disrupt neuronal progenitor migration?
Lisencephaly
Tublinopathies
What is associated with TGFbeta receptor mutations?
Mild MENTAL RETARDATION
Where are TGFbeta receptors expressed?
In the ventral zone of the developing cortex
What does TGFbeta do in vitro?
What happens in TGFbeta KO?
Initiates axons
In KO - doesn’t initiate axons
In a stripe assay with TGFbeta, where do axons/dendrites go/avoid?
Axons grow on TGFbeta
Dedrites AVOID TGFbeta
What does TBFbeta affect in the par complex?
Affects Par6
What are semaphorins?
What do they cause?
INHIBITORY guidance cues
Cause GROWTH CONE collapse
What are the different types of semaphorins? (examples)
Membrane-bound (in the retina)
Secreted (Sema3A)
Where is Sema3A expressed?
What does it do in the basal region?
In a GRADIENT from basal to apical
In the developing cortex:
Attracts dendrites
Suppresses axons
What does Sema3A do in stripe assay?
Promotes dendrite formation
Suppresses axon formation
Where do axons form in Sema3A stripe assay?
Where there is no Sema3A
How does Sema3A afect [cGMP] and [cAMP]?
How does this affect axon formation?
INCREASES [cGMP]
INHIBITS [cAMP]
Therefore INHIBTS PKA phosphoylation of LKB1
Allows the GSK3beta to function
Preventing axon formation
What does PKG affect levels of?
cGMP
What does cGMP do?
Promotes DENDRITE formation
What does cAMP do?
Promotes AXON formation
What are par complex proteins involved in?
Apical basal polarity
Crawling cells
Where is the Par3 complex localised?
In the LEADING EDGE
In the GROWTH CONE
How does TGFbeta affect the Par6 complex?
What does this cause?
Phosphorylates Par6
PROMOTES axon formation
Where does TGFbeta phosphorylate Par6?
APICALLY
What happens basally in the developing developing cortex?
Sema3A increases [cGMP] and inhibits [cAMP]
Inhibits phosphoylation of LKB1 and axon formation
Where is polarity inherited?
In the early cortex and in the retina
What happens to polarity in later stages of cortical development?
Transit amplifying cells produced with NO polarity (polarity lost during division)
Polarity must be re-established before neurite selection
