Nerve injury

1
Nerve injury Motoneurons

• Core concepts
• Loss of trophic support
• Excitotoxicity

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Causes of motoneuron death
NOCD
Early postnatal injury
Disease SMA ALS PMN
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Motoneuron disease

• 5000 affected in UK
• 1-2/100,000 new cases p.a.
• Affects men gt women
• Onset gt40 years of age
• 55-65 yrs old most commonly affected
• Genetic
• Not possible to prevent onset

• Symptoms
• Progressive muscle weakness wasting
• Hands, arms legs usually affected first
• May get spasticity, painful cramps loss of
  balance
• Affects vocal respiratory muscles

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Clinical syndromes

• Spinal Muscular Atrophy
• Hereditary condition defect in SMN gene
• (Chromosome 5q12.2-q13)
• Insufficient peripheral motor sprouting
• Increased motoneuron activity excitotoxicity?

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Clinical syndromesAge related motoneuron loss

• Senile Muscular Atrophy
• Affects 15 of elderly population
• 10-20 motoneuron loss
• Motoneurons show signs of damage
• ? CGRP, GAP43, p75
• ? trkB, trkC

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ALS

• Affects UMN/LMNs
• Mechanisms
• 10 Hereditary (SOD1)
• Oxidative stress
• EAA toxicity
• Glial EAAT2 abnormal
• ? Glutamate in CNS
• ? Glutamate in CSF
• NOS, Cox 2 induced
• Defect in RA pathway

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Goals of neuromuscular disease research

1. Prevent death
2. Maintain phenotype
3. Repair neuronal damage

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Trophic factors and motoneuron survival
BDNF, NT3, NT4 GDNF, NTN, PSN LIF, CNTF,
CT1 FGFs RA HGF IGF1
BDNF, NT3, NT4 GDNF LIF, CNTF FGF5, bFGF IGF1,
IGF2
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NOCD trophic factor knockout
NGF- trk A No motoneuron loss BDNF/NT4 - trk
B No motoneuron loss NT3 - trk C No motoneuron
loss p75 No motoneuron loss BDNF-NT4 double
KO No motoneuron loss trk B/C double KO No
motoneuron loss CNTF No motoneuron
loss CNTFR? 40-50 LOSS LIFR? LOSS gp130 40
LOSS GDNF- GFR?1 20-40 LOSS GFR?2 No
motoneuron loss c-RET Significant loss
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Motoneuron survival depends on age
P0 90 P3 80 P4 30 P5 0
and post-operative survival time
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Neonatal motoneuron death depends on lesion site

• Motoneuron loss also depends on duration of
  deprivation
• P0 axotomy 1 survival
• P0 crush 10-30 survival
• Delayed reinnervation P5 P10 crush 60
  survival

YES ?
? YES
Yes
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Is motoneuron death due to axon damage per se or
target deprivation?

• Motoneuron loss is regulated by target
  deprivation
• can be induced by NMJ blockade at birth
  (maintains MN immaturity)

injury induced release of glutamate (kills
immature MNs) Can be mimicked by NMDA injection
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Motoneuron death is regulated by target contact
Growing mode Transmitting mode
Muscle induced neuro-transmitter release
Ach
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Motoneuron death is regulated by target contact
Growing mode Preserved immature
state
Death by glutamate excito- toxicity
Prevention of neuromuscular interaction No
induced neuro-transmitter release
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Motoneuron maturation
Growing neurone transmitting
neurone
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Neonatal axotomy effects on surviving neurons

• Biochemical
• ? expression, Reg2, HSP27, GFR?1, p75, CGRP, CB,
  gp130, trkC, CNTFR?
• ? expression GAP43, c-Jun, NOS, NR1, NR2B, GAL,
  mRNAs for LIF, trkB, c-RET

• Physiological
• ? neuronal activity
• abnormal reflex patterns
• ? dendrite number, altered morphology

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Pharmacological manipulations that rescue dying
motoneurons

1. Neurotrophic support
2. Preventing excitotoxicity

• Important Outcomes
• Permanent survival
• Rescued motoneurons must reinnervate muscles
• Muscles must develop adequate force on
  reinnervation
• Spinal circuits must be re-established

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Neurotrophic support
1 week 2 weeks
combination nerve s.c
P3 sciatic cut single dose NTF treatment to
injury site
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Neonatal neurotrophic support

• Transient rescue
• BDNF lt 3 weeks
• NT3 lt 2 weeks
• NT4 lt 1week
• CNTF lt 2 weeks
• LIF lt 2 weeks
• GDNF
• combinations
• Restore ChAT levels

• Permanent rescue
• GDNF (AAV)
• Deleterious
• NGF (activity dependent)
• High dose BDNF
• receptor desensitisation or activity dependent

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Adult motoneuron death
YES ?
? NO/ YES -delayed
NO
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Adult nerve injury

• Loss of normal function
• Loss of reflex function
• Soma atrophy
• Motor c.v. ?
• ? ChAT
• ? transmitter receptors
• No cell death
• VRA
• Repeated nerve injury

• Regeneration
• ? GAP43, c-Jun
• ? CGRP, GAL,
• ? REG2
• ? HSP27
• ? c-Ret, GFR?1, LIFR, p75, CNTFR?, trkB
• ? gp130, ? trkC

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Exogenous NTFs reverse effects of injury
aid regeneration
? NGF, BDNF, GDNF, NT4

?
?trkB GFR?1 p75
Regeneration GAP43, CGRP Tubulin
?CNTF, ? NT3
?p75 ?? apoptosis in Schwann cells ? ?
regeneration
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Adult injuries that killRepeated nerve injury
? SA
EAA toxicity
? BDNF, GDNF, NT4

?
?trkB GAP43 p75
?
or ? Schwann cell derived trophic factors

• Immature state

Regeneration mode
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Adult injuries that kill Avulsion
Exogenous BDNF, GDNF prevents death
No transport of Schwann cell derived NTFs
EAA toxicity

?
?trkB GFR?1 p75 NOS
No access to Schwann cells
Regeneration possible ? GAP43, CGRP, Tubulin
Activation of p75 death pathway?
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Rescuing dying motoneurons Preventing
excitotoxicity

• Riluzole
• NOS inhibitors
• Glutamate inhibitors MK801

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(No Transcript)
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Clinical trials in ALS

• CNTF severe side effects fever, chest pains,
  muscle weakness, herpes virus activation
• BDNF Major side effects. Pain.
• IGF1 data not conclusive drug well tolerated
• GDNF, NTN not tested
• TCH346 (anti apoptotic) failed phase 2 trial
• Riluzole activity blocker
• Retinoic acid RALD2 RAR?

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References

• Lowrie MB Vrbova G 1992 TINS 15 80-84
• Greensmith L Vrbova G 1995 Neuro-muscular
  Disord 5 359-69
• Vejsada R et al 1995 EJN 7108-115
• Ma J et al. 2001 139 216-223