Lecture 32- Neurotrophins III Flashcards
What are the two forms of NGF? (paper 1)
-proNGF -mature NGF
What is the structure of NGF? (paper 1)
- has several parts -signaling peptide, pro-domain, furin-cleavage site, mature protein part
- when it has the pro-domain it is the pro form that is apoptotic
- when it is cleaved then it is the mature form that is anti-apoptotic
What NGF form is the more common? (paper1)
-the larger form, the proNGF -this is in the brain as well
How was the function of the pro-form of NGF discovered? (paper 1)
- make a mutant form of NGF that cannot be cleaved and see what happens
- the proNGF is pro-apoptic pro-neurotrophin
What is the function of proNGF? (paper 1)
-it is a pro-apoptotic pro-neurotrophin
What is the difference in binding affinity between pro-NGF and mature NGF? (paper 1)
-pro-NGF is a high affinity functional ligand for p75 receptor and does not bind nor does it activate TrkA -the proteolytically cleaved mature NGF is the preferred ligand for TrkA
What does NGF do to TrkA? (paper 1)
-NGF causes phosphorylation on TrkA receptor, proNGF does not phosphorylate thus does not activate TrkA, pro NGF cannot exert the effects that mature NGF has
What was the new perspective on the neurotrophin receptors after discovering the difference between the pro-form and the mature form? (paper 1)
-p75NTR signaling influences neuronal survival -proneurotrophin signaling -recruitment of cytoplasmic adaptor protein that transduce signals which ultimately regulates the transcription of genes invoved in apoptosis /PCD -extrinsic control of apoptosis -neurotrophins are produced as pro-neurotrophins bind to p75, can be cleaved and those mature neurotrophins then bind to the Trk receptors -so the two forms have different effects -increases tha capacity of neurotrophins to affect the cell survival and so on
What is the concept of reward and punishment? (paper 1)
- there is nuance, it is not that proneurotrophins are apoptotic and neurotrophins are antiapoptotic
- it is more reward and punishment model
- presence of mature BDNF= synapse would be strengthen, the neurite would connect the synapse is rewarded
- if have proBDNF= then the neurite retracts= punished
- reward and punishment is required for competition between multiply-innervating neurons at the NMJ
- pro-BDNF acting on p75NTR is the punisment signal
- mature BDNF is the reward signal
What is the idea of Hebbian synapse? (paper 1)
-“neurons that fire together, wire together” (Donald Hebb) - proposed that competition depends on relative timing of activation of synapses - originally proposed to explain learning and memory, but seems a more general phenomenon
What are the reward and punishment signals in neurotrophic signalling?
- proBDNF is the punishment signal
- mature BDNF is the reward signal
How does the reward and punishment work in practice?
- this has been shown to happen in LTP and LTD as well
- the environment in which the BDNF is released is important
- so depends on the enzymes present as well since they will cleave the proBDNF= then will have BDNF and will have promotion of the synapse
- if fewer enzymes will have more proBDNF and the synapse will not be strengthen
What is the yin and yang of neurotrophin action? (paper 1)
-Neurotrophins:
- 4 family members
- Expressed n two distinct forms
- Signal through two signalling systems
- Elicit opposing biological responses
- Trk receptors= pro, positive reward signals
- p75= the punishment signals, more pro death, negative
What is the paper 2 about?
-about what NGF is doing to promote survival of cells, what the precise mechanism is, what genes are turned on
What genes were turned on by NGF? (paper 2)
- in vivo assay
- Bax-/- mice show no developmental cell death
- Compare Bax-/- animals with and without NGF
- The effect of NGF on gene expression can be assessed
- Used a microarray to look at 34,000 mRNAs simultaneously
- Examine differences in NGF-regulated gene expression in the SCG
- double transgenic experiment, Bax= pro apoptotic factor so when knockout then the cell doesn’t die, as the Bax regulates the intrinsic signal of apoptosis
- the cells that saw NGF had many receptors for NGF
What is the ‘mystery mechanism’ of the TrkA signaling and cell survival? (paper 2)
-Hypothesis: “The winners kill the losers”
Characteristics of the ‘mystery mechanism’
- Kill neurons that have low TrkA signalling
- Be released from neurons with high TrkA signalling
- Not affect neurons with high TrkA signalling
- Start after TrkA signalling commences
- C is what happens in development
- prune back the neurotes according to the amount of growth ffactors at the target - Neurotrophin onto Trk receptors = cell survival
- Neurotrophin onto Trk + p75 receptors = better cell surviva
l -Neurotrophin onto p75 receptors = cell death!
What is this?
PIC5-all the cells expressing TrkA and p75 but not TrkB
-so you prune back the pro apoptotic effect
How is the TrkA activated and what does it mean? (paper 3)
- via an intracellular mechanism (Src and Fyn)
- selective activation of signalling pathways
- the non-NGF can cross blood brain barrier so maybe can be used to stop neurodegenerative disease?
What was the mechanism by which the BDNF did or did not promote myelination of neurons (paper 4)?
the mechanism was clear
- if BDNF saw p75= then schwann cell would myelinate
- if cell expresses TrkB= then inhibits myelination (weird as BDNF binds to TrkB)
What does a mature BDNF look like and what are the two important binding sites? (paper 4)
- BDNF binds to TrkB = one aspect and the other bit binding to p75
- these are different part of BDNF so maybe explains why there is a difference in function since it is a different part of the molecule
