Development 3 Flashcards
What is GDNF?
MOA?
Role?
Treatment?
Glial cell line-derived neurotrophic factor.
It is a member of a family of neurotrophic factors that include Neurturin, Artemin + Persephin.
GDNF preferentially binds to a cell surface receptor, GFRa1.
Binding induces dimerisation of RET (an RTK), activation of kinase domains, autophosphorylation of tyrosine residues, and activation of intracellular signalling pathways.
GDNF has a crucial role in the development + survival of neurones, particularly dopamine neurones.
ANd has been considered as a therapy for treating Parkinson’s disease where dopaminergic neurones in the substantia nigra die off.
What are the roles of trophic interactions in neuronal development?
Trophic interactions support the final stages of neuronal development after cells have been committed to a specific neuronal phenotype, migrated to their final destination and extended their axons to establish synaptic connections.
Trophic interactions support the survival of a subset of neurones by: supporting the formation and maintenance of appropriate synaptic connections, aswell as the growth of axonal/dendritic branches to support connections…
How does synapse formation allow neuronal survival?
Neurones are produced in excess during development.
They compete with each other for limited amounts of trophic factors (survival-promoting factors) secreted by their target tissues.
Neurones must project their axons towards the target tissue and require secreted trophic factors to survive.
Apoptosis eliminates neurones that have formed inappropriate connections = selection for neurones that have made correct connections.
Surviving neurones can then take up more trophic factors for survival.
What is an example of synapse formation regulating neuronal survival?
20,000 motor neurones are formed in spinal cord of chick, but only 10,000 survive.
Survival of motor neurones depends on establishing functional synapse with a single muscle cell, as even after NMJ formed, each muscle fibre can only be innervated by a single motor neurone.
How does neuronal survival dependence change during development?
Neurones are initially dependent on factors from glia and adhesion factors/guidance cues and are not dependent on target cells…
Only after axons reach their target and form synaptic connections, is there is a switch in dependence:
Neurones then depend on trophic factors for survival - or apoptosis if inappropriate.
What is transitory dependence?
Some neurones are required to innervate multiple targets or innervate a secondary target first prior to reaching the primary target.
Therefore, there is transitory dependence on a secondary target - for which final survival depends on reaching the primary target.
After reaching the primary target, pruning or loss of innervation with the secondary target can occur.
What are neurotrophins?
A family of trophic factors secreted by target cells that are essential for neuronal survival.
What is NGF?
How was it discovered + Sources of NGF?
Tumour sarcoma cells secreted a factor, NGF, promoting the growth of neurites.
Snake venom is a rich source of NGF.
Secreted by target cells + necessary for neuronal survival + neurite outgrowth.
= In the absence of NGF, atrophy of neurones.
NGF is a determinant of the size of a neuronal population. A limiting concentration of NGF causes neurones to compete for its uptake, only a proportion would survive.
How does NGF exert its action on neurites?
NGF exerts its actions locally, so can trigger neurite outgrowth without contact with the soma/cell body of the neurone.
What is the structure of NGF?
A polypeptide, secreted as a large precursor (pro-NGF).
Pro-NGF is processed intracellularly to give NGF.
NGF exists as a homodimer - consisting of 2 monomers (Cysteine knots), each containing 4 anti-parallel Beta strands connected by 4 loops and stabilised by disulfide bonds.
NGF acts at TrkA and p75 receptors as a homodimer.
How does NGF exert its effects via TrkA?
NGF binds as a homodimer to the large extracellular binding domain (d5) of TrkA (a receptor tyrosine kinase), which converts the external signal into an intracellular signal.
TrkA can exist as a dimer - NGF binding to d5 stabilises the TrkA dimer.
d5 is Ig-like.
Inducing structural changes that activate the intracellular TK domain for autophosphorylation of Tyrosine residues in each monomer.
TrkA signals via phospho-tyrosine multiple pathways including the ras/raf pathway.
- to promote rapid local effects or longer-term signalling events.
= induce cell survival, neurite outgrowth, neuronal differentiation and synaptic plasticity?
How does NGF signal ras/raf pathway?
phosphotyrosine residues on TrkA bind SH2 domains in Shc.
Shc is tyrosine phosphorylated + binds to SH2 domains in Grb2 adaptor protein.
Grb2 binds to Sos protein which links to membrane-bound GTPase Ras.
Binding induces exchange of GTP for GDP, causing activation of Ras.
Activated Ras recruits cytoplasmic Raf kinase.
Bound Raf kinase is activated and phosphorylates serine residues on on MEK.
MEK phosphorylates ERK, phosphorylated ERK can phosphorylate cytoplasmic targets for rapid local effects OR translocate into the nucleus to influence transcription factors for longer term signalling events by altering protein transcription.
How can NGF induce long term signalling changes?
Other than via Ras/Raf pathway + subsequent nuclear translocation of phospho-ERK…
NGF/TrkA can be internalised via signalling endosomes and transported retrograde to the nucleus in the cell body.
Where it can alter gene transcription + protein synthesis.
How does NGF interact with other receptors?
NGF has poor affinity for a second receptor, p75, which belongs to the TNF family.
However, proNGF has high affinity for p75.
The effects of interaction with p75 can vary:
p75 can promote growth cone collapse + apoptosis but also promote cell survival…
Signalling of p75 is distinct from TrkA, however, p75 can cooperate with TrkA to increase the affinity of NGF for TrkA.
Therefore, the outcome of NGF signalling depends on conc. of NGF (as limiting conc. = competition), which receptors = TrkA or p75…, and which signalling molecules are present.
Which neurones does NGF support?
NGF does not have widespread effects in the CNS, but does support a population of cholinergic neurones in dorsal root ganglia + sympathetic ganglia.
