Lecture 33: Neural Repair II

Question 1

Neurotrophic factors

Answer 1

Proteins (peptides) - support growth, differentiation, survival of developing and mature neurons

Key molecules (normal nervous system development & function) - keep important & connected neurons alive during development to adulthood, some require continuous factor signalling to survive

Many neurodegenerative diseases - >- one neurotrophic factors affected

Question 2

Three major neurotrophic factor types

Answer 2

Neurotrophins

Glial-cell line Derived Neurotrophic Factor (GDNF) Family of Ligands (GFLs)

Neuropoietic cytokines

Question 3

Neurotrophins
- define
- types in mammalian brain
- process

Answer 3

Peptide based molecules -> major survival signal for our neurons

Nerve growth factor (NGF), Brain Derived Neurotrophic Factor (BDNF), Neurotrophin 3 (NT3), Neurotrophin 4 (NT4)

Secreted as precursors -> cleaved proteolytically -> mature form -> dimerise (Hetero/homo) to bind to receptors

Question 4

What are the two main receptor types in neurotrophin signalling and their affinities?

Answer 4

1. Tropomyosin-related kinase (TrK) family of receptor tyrosine kinases (RTKs).
   - different affinity to different Neurotrophins
   TrK A - NGF, TrK B - BDNF & NT4, TrK C - NT3
2. p75 neurotrophin receptor
   - equal low affinity to all mature Neurotrophins
   - high affinity to pro-Neurotrophins

Question 5

Describe neurotrophin signalling

Answer 5

Homodimers bind to TrK receptors -> receptors dimerise & autophosphorylate by their receptor tyrosine kinase activity

• Local signal: neurite outgrowth
• Signal internalized and retrograde transported to cell soma by axons: gene expression changes -> cell survival, differentiation, neurite outgrowth

Question 6

Describe what happens when nerve growth factors bind to their receptors and what happens to them in AD.

Answer 6

NGF binding
- promote peripheral neurons survival
- promote central neurons survival (basal forebrain cholinergic neurons)

AD - basal forebrain cholinergic cell loss -> decrease hippocampus Ach levels

NGF & AD (experimental)
- injecting NGF intracerebroventricularly inhibited cholinergic cell apoptosis -> improvement
- trial stopped -> pain (diffusion to DRG NGF- responsive nocioceptive neurons via CSF) and hyperplasia (spinal cord, medulla)

Question 7

BDNF

Answer 7

Bind to TrKB with high affinity

High levels -> hippocampus, striatum, cerebral cortex

Cell survival (medium spiny neurons (MSN) requires it), synapse plasticity (learning & memory), neurogenesis (development 7 adult neurogenesis) involvement

Question 8

BDNF and HD
- normal BDNF function
- Mutant htt protein function
- mechanism (theorised) of selective MSN degeneration

Answer 8

• produced in other brain regions -> innervate striatum & anterograde transported to striatum
• downregulate BDNF -> affect transcription & axonal transport
• play part in degeneration
• increase proBDNF availability (striatum) -> send cell death signal (apoptosis)
• exact not known. theorised: combo of reduced trophic support (increase vulnerability) & toxic mutant htt action

Question 9

Therapies for HD and BDNF
what has been shown in mice

1. BDNF over-expression in transgenic mouse model of HD
2. BDNF over-expressing MSCs in transgenic mouse model of HD

Answer 9

1. Rescued striatum neurons -> promoted neurogenesis & differentiation into MSN lost in HD
   - Chemically induced/ viral vector of expression
2. Preclinical : transplant increased striatal neurogenesis, promoted their survival, improved motor skills
   Clinical: become first cell transplant therapy HD clinical trial

Question 10

GDNF family of ligands (GFL)

Answer 10

TGF-b superfamily

Secreted as precursors -> cleaved proteolytically -> mature form -> bind to ECM proteoglycans side chains (restrict diffusion & increase local conc.) -> dimerize to display effect

Question 11

Four main types and their receptors
(GFL dimers - GDNF-α receptors )

Answer 11

Glial-cell derived neurotrophic factor (GDNF) - GFRα1
Neurturin (NRTN) - GFRα2
Artermin (ARTN) - GFRα3
Persephin (PSPN) - GFRα4

Question 12

GFLs signalling

Answer 12

GFL-GFRα complex -> interact with 2 RET molecules (receptor tyrosine kinase) [tyrosine autophosphorylation ability] -> propagates intracellulaire signalling

Question 13

GDNF signalling and PD

Answer 13

GDNF & NRTN -> important: dopamine neuron survival

Trials for gene therapy introducing GDNF and NRTN -> promote DA neuron survival

Question 14

neuropoietic cytokines (Not examined)

Answer 14

Secreted protein from immune system
Important for neurogenesis and repair: IL-6, LIF, CNTF
Many unregulated in CNS when neuronal injury
Signal through gp130 receptor complexes through own specific receptor
Lead to gene expression that keep self-renewal, directs differentiation, cell survival

Question 15

Neuronal survival - other methods/clinical trials
Porcine choroid plexus transplant

Answer 15

Choroid plexus - organ produces most of CSF & secrete many neurotrophins into CSF

Preclinical: Transplant into PD animal models -> increased dopamine producing neurons improve motor effects without immunoreactivity