Neurotrophins

Question 1

What are neurotrophins?

Answer 1

Family of proteins which control survival and development of neurons by binding to specific receptors on the surface of neurons, leading to various cellular responses.

Question 2

How does the size of a target correlate with the amount of innervation?

Answer 2

• Fewer neurons innervate if there are less targets around
• extra target= more synapses/neurons
• Less target= less synapses/neurons

Question 3

What does this size/innervation correlation imply?

Answer 3

That the target is providing trophic support to cells (food) because otherwise the neurons wouldn’t go there

Question 4

Explain the neurotrophic factor hypothesis

Answer 4

• Make more neurons initially then eliminate the ones you don’t need
• Lots of neurons grow out, reach the target and the target release neurotrophic factors to promote survival
• More neurotrophic’s released from targets with a bigger tissue size
• When the tissue is removed there are more dying cells (e.g. neural death when limb bud removed)

Question 5

What protein regulates cell death in C.elegans?

Answer 5

Caspase 9

Question 6

What are the two classes of gene/protein involved in apoptosis and what do they mean?

Answer 6

Anti-apoptotic: inhibitor of cell death , e.g. Bcl-2. Loss of anti-apoptotic genes leads to lots of cell death

Pro-apoptotic: Executioner of cell death e.g. caspase 9, Loss of pro-apoptotic genes leads to excess cells

Question 7

What did Beuker suggests controls cell death through neural development?

Answer 7

Fast growing muscle cells might secrete survival factors
e.g. implanted sarcomas into chick embryos provoked selective survival of sensory and sympathetic neurons

Question 8

Who found NSF (Nerve growth factor) and how?

Answer 8

Rita, found it in snake venom and the submaxillary gland

Question 9

What is the structure of NSF?

Answer 9

• 2 alpha subunits, 2 gamma subunits, one beta subunit
• Beta is the important one
• NSF is synthesised as a precursor and then processed to the active form

Question 10

How is NSF shown to be used in long distance signalling using a campenot chamber?

Answer 10

• Campenot chamber allows neurons to be seeded in one chamber and neurites to extend to other chambers, meaning spatial separation of the neurons and axon.
• NSF placed in inner or outside chamber and cells will survive but if its placed in the middle the neurons on the outside wont survive as they have to grow out first and then reach the NGF
• Can also guide growth cones in vitro as they grow towards NSF

Question 11

What happens When NSF binds to the ends of growth cones?

Answer 11

gets ‘shipped back’ into the cell bodies - Retrograde transport
- Can be observed to be transported all the way back to the spinal chord!

Question 12

Explain NSF signalling

Answer 12

• NSF found on target cells, its receptor is TrkA which is found on the surface of neurons
• Binding, dimerisation, phosphorylation all occur activating the MAP kinase pathway and Akt which inhibits apoptosis
• Once NSF binds, it gets endocytosed into the neuron forming a signalling endosome which is transported to other regions of the cell e.g. cell bodies or dendrites

Question 13

How many other members of this neurotrophin family are there?

Answer 13

4, each with different receptors

Question 14

Which receptor do all of the neurotrophins in the NGF family bind to?

Answer 14

P75, can bind to the mature forms and the pre-cursors

Question 15

Why is the binding of the neurotophins to P75 context dependent?

Answer 15

• Mature NSF binds to p75 or trkA and promotes cell survival
• Precursor NSF binds to p75 and it causes cell death

Question 16

How do different classes of neurons have different dependencies and how do these change over time?

Answer 16

• Different classes of neurons depend on different neurotrophins e.g. Nodose ganglion neurons prefer BDNF
  Over time:
• Newly born neurons have no dependency
• NT3 (neurotrophin) supports neurons in early development
• Arrival at target may coincide with new expression of neutrophic factors by the target

Question 17

What is the purpose of cell death in these cases?

Answer 17

• ‘Size control’
• Removal of transient structures (die when job done)
• Pruning (rid of cell parts rather than cell death)

Question 18

How can we use a campenot chamber to observe the effects of cells deprived of NGF?

Answer 18

• Separation of cell bodies from axons in the campenot chamber allows axon-specific NGF-deprivation
• Caspase6 activation can be seen in NGF-deprived sensory axons
• Causes apoptosis/ pruning

Question 19

What is an example of how in different conditions, NGF can cause apoptosis?

Answer 19

• NGF induces shedding of Amyloid precursor protein which is on the surface of growing neurons
• APP gets released and activates ‘death receptor 6’ - a member of the Tumour necrosis family (TNF)
• This receptor activates caspase 6

Question 20

Why is p75 a ‘dependence receptor’

Answer 20

initiates cell death in the absence of a ligand

Question 21

What do members of the tumour necrosis family share with p75?

Answer 21

an intracellular death domain which can directly initiate caspase-mediated programmed cell death