Neurotrophins Flashcards
What are trophic factors?
Factors made by neuronal target which influence neuronal/synaptic survival
What did Vicktor Hamburger do and what did this show?
Removed the limb bud - resulted in fewer motor neurons in the ventral horn and fewer DRG neurons on the side of removal
Added in an extra limb bud - extra muscle, more motor and sensory neuron survival
If there is a larger target, what influence does this have on the number of neurons and synapses?
Increases it
Why are there differences in neuron number when limb buds are added or removed and why?
Due to changes in cell death
Programmes cell death matches the number of neurons to the size of the target field
What did Bueker hypothesise?
Trophic survival factors are secreted by fast-growing muscle-like cells
What experiment did Bueker do?
What did this show?
- Took a sarcoma cell like derived from a muscle tumor cell and placed it into a chick embryo near the spinal cord
- This increased the amount of neurons IN THAT AREA
- Then, took the sarcoma cell GENERALLY in the egg - so that the factors produces (trophic factors)
- This had a general effect on all of the neurons (even when there was no direct contact between the tumor and the embryo)
- Implanted sarcoma provoked selective survival of non-placodal sensory and sympathetic neurons
- Demonstrated the presence of a diffusible growth factor
When raise antibodies against NGF (nerve growth factor) what happens?
Blockage of DRG growth
Do ALL sarcomas promote neuronal survival?
No
Where can NGF be purified from?
Sarcoma tissue
Muscle
Snake venom
Submaxillary gland
What is the structure of 7S-NGF?
- Protein complex
- 2 alpha, 2 gamma and one beta subunit (a2by2)
In NGF, what is the importance of the beta subunit and what is the structure of it?
Active component of the complex
Dimer
In NGF, what is the importance of the alpha and gamma subunits and where are they found?
Storage complex of the beta subunit
Only found in the submaxillary glad
What are the levels of trophic factors proportional to?
The size of the target
Where produces neurotrophic molecules and what effect does this have on nerves?
Muscles - promotes the survival of the neurons that innervate them
What can NGF do?
Promote cell outgrowth
Promotes cell survival
Guide growth cones in vitro (chemoattractant)
How does NGF signalling occur, what does this mean?
NGF and NGF receptor is internalised and undergoes retrograde transport from the growth cone to the cell body
DRG cell bodies can take up NGF, even when it is only applied to their axons
What is the high affinity NGF receptor?
What is the low affinity NGF receptor?
High affinity: TrkA
Low affinity: P75-NTR
What is the TrkA?
A receptor tyrosine kinase
What happens when the TrkA is activated?
- Dimerisation
- Autophosphoylation
- Activates CREB, which binds to DNA and causes transcription
What does the TrkA affect when it is activated?
Survival
Differentiation
Growth
Movement
What does the P75-NTR NGF receptor affect when it is activated?
Promotes cell death or cell survival, depending on context
Which family is NGF a part of?
Neurotophin family
What are other neurotrophins, as well as NGF?
Brain-derived growth factor (BDGF)
NT3 and NT4/5
Why is it hard to identify neurotrophins?
Present in low levels in the nervous system
Why are neurotrophins present in low levels in the nervous system?
So they can SELECTIVELY chose which neurons survive
If high levels - get inappropriate neuron growth
What are NT made as?
What does this mean?
Pro-proteins
Have parts which must be cleaved in order for them to become active
Which NT bind to which receptors?
NGF to TrkA
BDNF and NT 4/5 to TrkB
NT3 to TrkC
All can bind to P75
Which receptor can the pro-protein of a NT bind to?
P75
What is the activation of the different NT receptors required for?
Survival of DIFFERENT SETS of neurons
How are the NT receptors expressed in the body and why?
Differently in different neurons as different neuron types are dependant on different NT
Where are the receptors for NT found and what do they respond to?
On neurons, respond to NT released from muscles
Can NT have an effect on non-neuronal tissue?
Yes
Which cells respond to combinations of NTs?
Merkel cells
Ruffini afferents
Early in development, what allows neurons to send projections to their targets?
They either have NO neurotrophic dependancy, or are dependant on NT3, which they make themselves
Once a neuron arrives at its target, what happens to NT expression and response?
Target expresses a new NT, which switches the dependancy of the neuron to whatever the target is making
Which organisms have no neurotrophins?
C.elegans and drosophila
What are other families which support neuron survival?
1) Glia - derived neurotrophic factors
2) Cytokines
3) Testosterone
What do GDNF support?
Midbrain dopamingeric neurons
AND
Polysynaptic neuron survival (when released from muscle target)
What are cytokines important in?
The immune system
Examples of cytokines?
Ciliary neurotrophic factors (CNTF)
Hepatocyte GF
Macrophage-stimulating protein (MSP)
What does testosterone influence?
- The size of the motor neuron pool which accounts for sex differences
- More motor neurons in this area in males
When axons get to their targets, what are target-derived factors critical in? (3 things)
The formation of monosynaptic stretch reflex between motor neurons and proprioceptive Ia sensory neurons (PN)
AND
The determination of dendritic morphology of neurons and connectivity
AND
Neuron survival
AND
Feedback to proprioceptors
Where do monosynaptic neurons receive direct input from and how is this different to other neurons to other muscles?
Directly from proprioceptive neurons (no interneurons)
Different to other neurons, which are polysynaptic (have interneurons between them and the incoming proprioceptors
What are proprioceptive neurons?
Sensory neurons
What are polysynaptic neurons dependant upon, what does this do?
- GDNF (glia-derived neurotrophic factors) from the muscle target
- Turns on expression of Pea3
In a KO mouse of Pea3, what happens to the polysynaptic neurons?
Now have the same dendritic morphology as the monosynaptic neurons and now innervate the muscle of the monosynaptic neurons
What connections with the proprioceptive neurons do MN innervating the triceps and pectoral muscles develop?
Monosynaptic
What connections with the proprioceptive neurons do MN innervating the cutaneous maximus and latisimus dorsi muscles develop?
Polysnaptic
Where is GDNF secreted from and what does this do?
From cutaneous maximus and lattisimus dorsi
Turns on the expression of Pea3 in motor neurons
What does circuit completion rely on?
Target feedback
What does muscle-expressed NT3 do?
Induces the expression of Er81 transcription factor in the Ia proprioceptors (which create synapses with the motor neurons innervating the muscle)
What does KO of Er81 cause?
What does this show?
Failure of Ia central projection to reach the ventral horn and from monosynaptic connections with the appropriate MNs
No synaptic loop occuring
Shows Ia neurons are dependant on finding the target and turning on Er81
(dependant on FEEDBACK from the target)
What does target feedback determine?
Final patterns of dendritic and axonal connections
Why do some neurons themselves make NTs?
To help get neurons to their targets
To support the synapses of other neurons synapsing onto them
If a neuron target is another neuron, what is the connection regulated by?
NT factors
Why are NT anterograded?
To support the survival of downstream neurons
What is anterograde transport?
Transport from the cell body to the periphery
What is retrograde transport?
Transport from the periphery to the cell body
Why do early DRGs release NT3?
To help neurons get to their targets
What is the primary determinant of synaptic survival?
Coordinated electrical activity of the pre and post synaptic cell
What does non coordinated electrical activity of synapses cause?
Weakening of synapses and reduces survival of the neurons
What happens if an egg is treated with curare and why?
Greater motor neuron survival, due to the blockage of the nicotinic AchR
Blocking activity stops the initiation of cell death mechanism (doesn’t give the chance of the 2 neurons to no fire in a coordinated way)
How is a single muscle fibre innervated early on?
By many motor neurons
Why do we only want one MN to innervate a single muscle fibre?
To reduce to confusion of inputs
So that the MN fires at the same time as the muscle contracts
How are the number of MN innervating a single muscle fibre reduced?
Which MN survives?
By competition between the MNs
- Best connected MN survives (synchronous firing and supporting NTs)
- This connection will get stronger and the other (asynchronous) connections will get weaker
What is affected by electrical activity? (2 things)
Cell survival
AND
Synaptic strength
What happens to synapses if electrical activity is blocked?
Reduction in synaptic loss
More polyneuronal innervated muscle fibres
What does deliberate non-coordinated electrical activity cause?
Increase rate of synapse loss
What converts electrical activity into survival?
Neurotrophins
The more active a synapse - the more neurotrophin it takes up by membrane recycling
How does NT relate to target tissue mass?
The greater the target mass, the more NT available
What type of molecules are neurotrophic factors?
Growth factors
Which other neurotrophin has a high homology with NGF?
BDNF (Brain-derived neurotrophic factor)
What is the difference between neurotrophic factors and neurotrophins?
Neurotrophic factors are a generalisation of molecules which promote cell survival and include:
- Cytokines
- Glial-derived neurotrophic factors
- Testosterone
- Neurotrophins
Nerurotrophins are a class of neurotrophic molecules and include:
- NGF
- BDNF
- NT3
- NT4/5
What direction do NT support go in?
All directions:
- From target to innervating neuron
- From innervating neuron to target
How can neuronal survival be changed experimentally?
Altering electrical activity
What neurons are muscle spindles (cells) innervated by?
Why?
Motor and sensory
Sensory fibres send information to the brain about the amount of stretch happening in the muscle fibre
