Neurotrophic Factors

Question 1

Neurotrophic factors are proteins which promote

Answer 1

• growth
• differentiation
• survival
  of neurons (peripheral and CNS)
• essential for formation of mature neuronal circuits
• originate from target tissues
• very potent molecules

Question 2

many different families

Answer 2

Neurotrophins

• nerve growth factor (NGF)
• brain derived neurotrophic factor (BDNF)
• neurotrophin-3 (N-3)
• neurotrophin-4/5 (NT-4/5)

CNTF family

• cilliary neurotrophic factor (CNTF)
• leukaemia inhibitory factor (LIF)

GDNF family

• gilial-derived neurotrophic factor (GDNF)
• persephin

Fibroblast growth factors

• Acidic FGF (aFGF)
• Basic FGF (bFGF)
• FGF-5
• FGF-9

Insulin-like growth factors

• IGF-I
• IGF-II

Question 3

roles in development

Answer 3

• neurons require trophic support (trophic factors) to be available for survival and maintenance of appropriate target connections
• target tissues synthesise the appropriate trophic factors and make them available to developing neurons
• target tissues synthesise trophic factors in limited amounts - survival depends on neuronal competition

Question 4

neurotrophic factor hypothesis

Answer 4

• neurons extend axons to target
• targets secrerte neurotrophic factor
• bind to receptors, signal etc and stay alive
• if they don’t receive neurotrophic factor - cells die

neurons die from lack of trophic support

Question 5

NGF

Answer 5

Nerve growth factor

• secreted protein
• promotes survival of neurons
  
  • sympathetic, sensory, CNS
  • important for neuronal outgrowth

Question 6

BDNF

Answer 6

Brain derived neurotrophic factor

• related sequence homology with NGF
• involved in nearly all stages of brain circuit development:
  
  • stem cell survival
  • neurogenesis
  • differentiation
  • neuronal guidance
  • branching and survival of differentiated neurons
  • formation and maturation of spines and synapses

Question 7

Where do neurotrophins come from?

Answer 7

target tissue derived

• NGF in targets of sympathetic neurons
• BDNF in muscle

other paracrine sources

• BDNF in schwann cells after peripheral axonal lesion
• can also be made in astrocytes

autocrine
- BDNF in DRG neurons promote DRG neuron survival

Question 8

Neurotrophin receptors

types

Answer 8

• High affinity = Trk receptors
  
  • specific for the individual neurotrophin
    
    • NGF = TrkA
    • BDNF+NT4/5 = TrkB
    • NT3 = TrkC
• Low affinity = p75 receptor
  
  • common for all neurotrophin family members

Question 9

Neurotrophin receptors

features

Answer 9

TrkA, TrkB, TrkC receptors

• all have extracellular ligand binding domain
• single transmembrane spanning domain
• cytosolic region has tyrosine kinase activity
• binding of neurotrophin leads to signal

p75 receptor
- binds to all neurotrophins
- homologous to CD400 and TNF receptor
function? - involved in intrinsic cell death pathways?
- signalling pathways?

Question 10

Neurotrophin receptors

location

Answer 10

• TrkA mRNA - localised to selected PNS/CNS sites - suggests restricted activities for NGF
• TrkB and TrkC and their ligands - much wider distribution in CNS - suggestion BDNF and NT3 have more diverse actions than NGF
• TrkB and TrkC exist in truncated forms - no cytoplasmic domain, i.e. no signalling element and truncated TrkB and TrkC are widespread in CNS, e.g. on astrocytes

Question 11

NGF roles

Answer 11

• dorsal root ganglion (sensory) neuron survival at birth
• sympathetic neuron survival/development?
• nociceptive neuron survival
• learning and memory
• neuroprotective in animal models
• overexpression of NGF –> high sensitivity to pain

confirmed through knockout of NGF and TrkA

Question 12

BDNF roles

Answer 12

• lack of TrkB - not able to feed and die
• lack of BDNF - head bobbing/spinning, 90% loss of vestibular ganglion neurons, no axons innervating vestibular receptors - all require BDNF
• lack of TrkB - significant loss of motorneurons (but just a lack of BDNF doesn’t cause motor neuron loss)
• important for synaptic plasticity
• important therapeutic target as BDNF is important for normal synaptic function - could treat a lot of different brain disorders by increasing synaptic plasticity - e.g. schizophrenia, depression

Question 13

NT3 and NT4/5

Answer 13

• largest sensory neurons - afferents from muscle spindle - NT3 dependent
• NT3 and TrkC knockout = abnormal posture/movements - due to large fibre sensory neuropathy
• NT4/5 knockout - some effects on memory (hippocampus and amygdala)

Question 14

Trk Activation

Answer 14

• single transmembrane domain
• neurotrophin binds
• dimerisation of receptors
• activates tyrosine kinase domain
• phosphorylation of tyrosine residues
• recruitment of cytosolic proteins (e.g. phospholipase C)
• activation of downstream signalling

Question 15

p75 activation

Answer 15

possible signalling molecules
- ceramide, JNK, NFkB

p75 knockout mice

• loss of sympathetic and sensory neurons
• increased cholinergic neurons in basal forebrain

Can increase neuronal expression in different areas too, just because it is involved in cell death doesn’t mean it is all bad, it might actually be really important during certain stages of development

Question 16

p75 receptor features

Answer 16

• p75 binds each of the neurotrophins with similar affinity, as well as other ligands
• lacks intracellular kinase domain (unlike Trk)
• probably involved in cell death and inflammation (unlike Trk)
• beta-amyloid peptide binds to p75, a mechanism that might contribute to death of cholinergic brainstem neurons in AD
• p75 can improve or worsen chance of cell survival
• p75 might enhance ligand binding to Trks

p75 - putative mediator off cell death

• p75 is homologous to TNF receptor family which contain a death domain
• p75 promotes apoptosis in several developing neural populations
• p75 knockout mice show reduced cell death in retinal, cholinergic neurons
• in cultured sympathetic neurons (which express TrkA and p75), BDNF seems to induce cell death by activating p75

Question 17

pro-neurotrophins

Answer 17

• precursors for neurotrophins
• 30-35 kDa proteins are cleaved to produce mature neurotrophins
• pro-neurptrophins have effects on their own
• can be secreted and then act via p75 receptor activation
• proNGF, proBDNF - can cause apoptosis
• mature neurotrophins have other actions

Question 18

CNTF

Answer 18

Ciliary Neurotrophic Factor

• 24kD protein
• related to interleukin-6, leukaemia inhibitory factor
• promotes motor neuron, sympathetic neuron survival
• is not found in neuronal target tissues (present in schwann cells)
• is not released - acts as a lesion factor
• knockout results in neurodegeneration
• receptors - no intrinsic tyrosine kinase domain, recruits tyrosine kinase after binding to receptor

Question 19

IGFs

Answer 19

Insulin-like growth factors

• 7kD protein
• structural homology to insulin
• made in liver, muscle, bone, also locally in brain
• overexpression increases brain size and myelination, increased neurogenesis, synaptogenesis
• knockout - hypomyelination, reduced neuronal numbers
• high brain expression pre and post natal periods
• protective actions - signals via PI3K, proliferative actions - via MAPK pathways
• IGFs produced in periphery can actually enter the brain (not the case for other neurotrophins)

Question 20

Therapeutic potential

Answer 20

Neurodegenerative diseases/acute brain injuries

• ALS
• PD
• AD
• Sensory neuropathies (drug, diabetes, HIV related)
• stroke
• spinal cord injury

very successful in cell and animal studies but not translated well into humans

Question 21

Neurotrophic factors side effects

Answer 21

Large does used because you can’t get much in to brain - therefore lots of side effects in periphery

• hallucinations
• nausea
• weight loss
• local inflammation
• fever
• muscle pain

Question 22

How to overcome problems of administering neurotrophic factors

Answer 22

• intrathecal delivery (spinal cord)
• pump systems (infusions)
• implants (slowly release delivery - mimics what happens normally)
• small molecules (many antidepressants) (use small molecules to increase neurotrophin expression, antidepressants used to increase BDNF levels)
• agents which may increase neurotrophic factors (e.g. exercise, antidepressants)
• viral vectors to deliver neurotrophic factors
• cell transplantation - to provide these factors (e.g. stem cell therapies))