Neurotrophins Flashcards
neutrophil hypothesis
neurone survival depends on a supply of neurotrophic factors synthesised in limiting amounts in their target fields
stages in neurodevelopment
- determination
- proliferation
- migration
- axon elongation
- synapse formation
- synapse rearrangement
trophic interactions support which stages?
survival of subset of neurones
formation + maintenance of appropriate connections
growth of axonal + dendritic branches to support the connections
neurotrophins
- features
- best studied example
family of trophic factors for neuronal survival
secreted by target cells
(neurones or innervated tissues)
e.g. Nerve Growth Factor
nerve growth factor (NGF)
- discovery
in a limb bud experiment:
implanted some tumour cells as a control
-> one type of tumour caused growth of nerves toward tumour
.:. tumour secreted an attracting factor
used snake venom (phosphodiesterase source) to destroy any Ras
-> massive neurone outgrowth
.:. snake venom = rich in NGF sources
-> then purified NGF
NGF in vivo
- effects on cervical ganglia
if treated daily with NGF anti-serum
-> atrophy + unhealthy
+ loss of neurones
control
-> sustained connections + survival of neurones
NGF in vivo
- neurone population size
inject extra NGF into immature animals
-> enlargement of sympathetic ganglia
NGF can determine size of a neuronal population
action of NGF is concentration-dependent
limiting conc of NGF
-> neurones compete for it
-> only a proportion survive
= rest die
only enough NGF produced to promote survival of required no. of neurones
NGF exerts a local action on neurites
neurites grow if NGF source present
neurites regress if NGF then removed
contact with neuronal cell body NOT required
NGF structure
polypeptide ~118 aa
dimer
secreted as a larger precursor
-> processed extracellularly to give NGF
each NGF monomer
= 4 anti-parallel beta strands connected by 4 loops
b strands connected by 3 cysteine disulphide bonds
= a ‘cysteine knot’
NGF and TrkA
NGF binds to d5 immunoglobulin domains on TrkA extracellular domain
-> activates intracellular Tk domain
NGF and dimeric TrkA
binding dimeric NGF stablises dimeric TrkA
- > induces structural changes
- > activates TK by autophosphorylation
crystal structure of NGF in complex with d5 on TrkA
NGF core binds to C terminus on d5
NGF N terminus binds to ABED sheets of d5
activated TrkA
- promotes signalling pathways
PI3 kinases
-> cell survival
Ras/Raf pathway
-> neurone outgrowth + neuronal differentiation
PLC
-> activity-dependent plasticity
how does TrkA interact with multiple targets?
(after autophosphorylation)
via phosphotyrosine
->
activity-dependent plasticity
synapses strengthened or remade depending on amount of nerve activity
ras-raf pathway
- up to activating Ras
- phosphoTyr residues on TrkA bound by SH2 domains in SHC protein
- SHC is Tyr phosphorylated
- > bound by SH2 domains in ‘adaptor protein’ Grb2 - Grb2 binds Sos protein which links to membrane-bound Ras
- binding promotes exchange of GTP for GDP
= activates Ras
ras-raf pathway
- post active Ras
- active ras recruits cytoplasmic Raf kinase
- activates bound Raf
- > phopshorylates serine residues on MEK - MEK phosphorylates ERK
- phosphoERK phosphorylates multiple cytoplasmic targets OR migrates to nucleus to influence TFs
retrograde transport of NGF-TrkA signalling
NGF-TrkA complex taken up by neurones
-> trafficked to cell body
internalisation via endosome formation
p75NTF
- features
2nd receptor of NGF
balances survival-vs-death decisions
low affinity for processed NGF
- higher affinity for unprocessed form
associated with neurone apoptosis
p75NTF
- mechanism
couples to multiple signalling pathways
- different to TrkA signalling cascades:
> RhoA
-> growth cone collapse (via apoptosis)
> c-Jun
-> cell death (apoptosis)
> NF-kB
-> cell survival
outcome of NGF signalling depends on..
NGF concentration
which receptors are present
intracellular signalling molecules expressed + in vicinity of receptors
longer term changes from internalised + trafficked NGF-TrkA
NGF doesn’t support all neurones
doesn’t have widespread effects in CNS
does support population of cholinergic neurones in basal forebrain
brain-derived neurotrophic factor (BDNF)
structurally homologous to NGF (50% sequence identity)
released as a precursor proBDNF peptide
- > processed into BDNF
- > binds to TrkB
(proBDNF binds to p75 receptor)
neurotrophins
- and Trk receptors
- and p75 receptor
Try receptors are homologous
- distinct but overlapping specificities
TrkA - sensory + sympathetic neurones
TrkB and TrkC - more widely distributed
all neurotrophins share same p75 receptor
which neurotrophins bind to which:
TrkA?
TrkB?
TrkC?
NGF
BDNF
NT-4/5
NT3 (weakly)
NT3
neurotrophins collaborating with other players in neuronal development and maintenance
- example
Slit1 + BDNF + Notch
- encourage branching of dendrites in brain
Glial-derived neurotrophic factor (GDNF)
- features
40kDa dimer
preferentially binds to GFR-alpha1
crucial role in development + survival of various neurone populations
- notably dopamine neurones
GDNF
- Parkinson’s disease
therapeutic candidate
involved in survival of dopamine neurones
- die in Parkinson’s disease
