Lecture 10 - Neurotrophic Factors I Flashcards
Developmental process— Neurulation
(Folding process in vertebrate embryos -transformation of the neural plate into
the neural tube.)
What determines differentiation/formation
of neural tissues?
Phylogenetically similar
Neural tube — forms the central nervous system
Induction & patterning are guided by:
* Intrinsic factor: Molecules induced in cells upon exposure to an inducing factor.
* Extrinsic factors: signaling molecules provided by other cells.
* Local events: contact and diffusible factors.
Thus, cells must be “competent’ and precisely timed to provide and receive signals. A complex multistep process.
How is phenotype controlled by signals from the neuronal target, using sympathetic neurons as a model?
Sympathetic neurons
* Most produce norepinephrine as transmitter.
* But those innervating sweat glands in foot pads uses acetylcholine.
How can this arise? - switching of biosynthetic machinery
Early observations (1920s-40s)
Transplantation of target organ
* Interpretation: factor/s produced by the target tissue increase the number of neuron or prevent the number of neurons from dying naturally.
* Target tissue influence the number of neuron in mature animal
Neurotrophic factor hypothesis
* Developing neurons compete with each other for a limited supply of neurotrophic factor/s (NTF) provided by the target tissue.
* Successful competitors survive; unsuccessful ones die.
What was the evidence that NGF affects sympathetic neuron development?
Administration of anti-NGF to new born mice
Loss of neuron in ganglia and
Complete disappearance of sympathetic chain of ganglia.
When deprived of NGF for 3-5 days.
Neuroactive factors: affecting survival of neurons
What are examples of some factors?
Neurotrophins
NERVE GROWTH FACTOR
NEUROTROPHIN-3
BRAIN-DERIVED NEUROTROPHIC FACTOR
NEUROTROPHIN-4/5
GDNF family of ligands (GFL)
GLIAL CELL-LINE DERIVED NEUROTROPHIC FACTOR (GDNF)
NEURTURIN (NTN)
PERSEPHIN (PSP)
ARTERMIN (ART)
Neurotrophic factors (NGF, BDNF, NT3, NT4/5)
What are their effects?
Classical effects — neuronal cell survival
Others:
Regulate axonal/dendritic growth and guidance.
Synaptic structures/connections
Neurotransmitter release
LTP
Synaptic plasticity.
Development of other non-neuronal organs.
Involvements:
Glial biology.
Vascular formation.
Tumor progression.
Dorsal root ganglia — sensory ganglion derived from neural crest
What is their function?
Cells involved in sensing pain, touch, pressure.
Cells express peptides, small molecules as neurotransmitters.
Some lessons from knockout studies
* Homozygous knock-out of neurotrophins (NGF and NT 1-4) — lethal mutations.
* Haploinsufficiency — (reduced amount in +/- mice) — many other deficiencies…
Neurotrophic factors requirement may be switched in the lifetime of the organ
What is an example?
- At birth (day1-P1) - 80% neuron express TrkA
- As DRG develops into adult, the number of neuron expressing TrkA receptor decreases.
- Dependency of NGF is decreased with development in small diameter neuron (IB4+ cells).
- Remove NGF(<2 d) — decrease nociceptive neurons and hypoalgesia
GDNF is the dependent factor when DRG cells develop to maturity
How was this found?
- A subpopulation of DRG cells (IB4+) switch growth factor dependence from NGF in early development to GDNF.
- Most IB4 cells express Ret (GDNF receptor system) in late development.
Phenotypic changes when NGF deprived 4-11 days — - No. of DRG cells - unchanged.
- Increasing Ret-expressing population of nociceptors differentiates into 3 subclasses of neurons expressing specific Mas-related G-protein-coupled receptors (Mrgpr) - MrgprA3; MrgprB4; MrgprD
What is the first neurotrophin discovered?
NGF is the first Neurotrophin discovered
4 others
Brain derived neurotrophic factor (BDNF), Neurotrophin 3 (NT 3), Neurotrophin 4/5 (NT4).
Phylogenetics: ligand and receptor found in lower animals- snail, frogs, avian, fish, zebra fish, fugu. snake But not in flies or c. elegans.
What are the features of neurotrophins?
Neurotrophins associate into non-covalent dimers
Neurotrophin-Receptor interactions
* Neurotrophins bind to TrK (tropomyosin-related kinase)
* Ratio of binding: 2:2 (neurotrophins: Trk)
Distinct contact sites of Trk binding with neurotrophins
* Shows high identities between different Trk — involve in direct binding of ligands.
What are the features of TrK?
TrK biogenesis and trafficking
* TrkA-containing endosomes in neurons are traffic in anterograde & retrograde
* NGF-TrkA binding results in internalization of ligand-receptor complex
Internalized NGF-TrkA have activities — Endosomal signalling
* Signalling is prolonged.
* Retrograde traffic is critical.
Trk can interact with other membrane components
Early studies
* TrK show specificity of ligand interactions and affinity to receptor is increased in the presence of a co-receptor, p75
No evidence of TrkA-p75 directly binding to each other
Free flow in membrane to partner with more
What are some interesting and puzzling observations regarding Trk and NGF?
How can they happen?
- p75 can enhance neurotrophin binding to Trk — increase affinity and specificity of ligand-Trk interactions.
- High doses of mature neurotrophin elicit cell death through p75NTR independent of Trk.
- Activation of p75NTR can also induce apoptosis in oligodendrocytes, neurons, and vascular smooth muscle cells when Trk activation is reduced or absent.
How can this happen?
* Tissue specific expression of multiple NGF forms
*Multiple isoforms of NGF with Mw 13.5 — 30 Kd —due to different processing of the proprotein.
NGF can bind to p75 directly too but not high affinity. Why?
- p75NTR also known as Low-Affinity Nerve Growth Factor Receptor, LNGFR (nerve growth factor receptor (TNFR superfamily, member 16)
- Note- contact sites are similar with all NT — thus, p75 binds all.
- However, proNGF (proNT) can bind to p75 with high affinity in the presence of yet another co-receptor, Sortilin.
What is Sortolin?
Sortilin: prototype of the Vps10p domain receptor family
Involve in both
* Cell Signalling
* Intracellular protein sorting
Sortolin plays important roles in TrkA neuronal trafficking
Proneurotrophins can bind to co-receptors directly
What is the outcome of the different partnerships?
Sortilin > proneurotrophins > p75NTR > death
Sortilin > neurotrophins > Trk > survival
p75NTR > neurotrophins > Trk > survival
