Nerve injury &

Motoneurons

Core concepts:•Loss of trophic support•Excitotoxicity

Causes of motoneuron death

NOCD Early postnatal

injury

DiseaseSMAALSPMN

Motoneuron disease• ~5000 affected in UK

– 1-2/100,000 new cases p.a.

• Affects men > women• Onset >40 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

Clinical syndromes:

Spinal Muscular Atrophy• Hereditary condition defect

in SMN gene

(Chromosome 5q12.2-q13)• Insufficient peripheral motor

sprouting• Increased motoneuron

activity – excitotoxicity?

Clinical syndromes:Age 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;

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

Goals of neuromuscular disease research

1. Prevent death

2. Maintain phenotype

3. Repair neuronal damage

BDNF, NT3, NT4GDNFLIF, CNTFFGF5, bFGFIGF1, IGF2

BDNF, NT3, NT4GDNF, NTN, PSNLIF, CNTF, CT1FGFs RAHGFIGF1

Trophic factors and motoneuron

survival

NOCD & trophic factor knockoutNGF- trk A No motoneuron lossBDNF/NT4 - trk B No motoneuron lossNT3 - trk C No motoneuron lossp75 No motoneuron lossBDNF-NT4 double KO No motoneuron losstrk B/C double KO No motoneuron loss

CNTF No motoneuron lossCNTFR 40-50% LOSSLIFR LOSSgp130 40% LOSS

GDNF- GFR1 20-40% LOSSGFR2 No motoneuron lossc-RET Significant loss

Motoneuron survival depends on age

and post-operative survival time

P0 90%P3 80%P4 30%P5 0%

YES

YES

Neonatal motoneuron deathdepends on lesion site

Yes

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

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

Muscle induced neuro-

transmitter release

Motoneuron death is regulated by target contact

Growing mode Transmitting mode

Ach

Prevention of neuromuscular

interaction

No induced neuro-

transmitter release

Motoneuron death is regulated by target contact

Growing mode Preserved immature state

Deathby

glutamateexcito-toxicity

Motoneuron maturationGrowing neurone transmitting neurone

Neonatal axotomy: effects on surviving neurons

Biochemical expression, Reg2,

HSP27, GFR1, 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

Pharmacological manipulations that rescue dying motoneurons

1. Neurotrophic support

2. Preventing excitotoxicity

Important Outcomes

1. Permanent survival2. Rescued motoneurons

must reinnervate muscles

3. Muscles must develop adequate force on reinnervation

4. Spinal circuits must be re-established

Neurotrophic support1 week 2 weeks combination

nerve + s.c

P3 sciatic cut + single dose NTF treatment to injury site

Neonatal neurotrophic support

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

Permanent rescue• GDNF (AAV)

Deleterious• NGF (activity dependent)

• High dose BDNF– receptor desensitisation or

activity dependent

NO/

YES -delayed

YES

Adult motoneuron death

NO

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, GFR1,

LIFR, p75, CNTFR, trkB

gp130, trkC

trkBGFR1

p75

NGF, BDNF, GDNF, NT4

***

CNTF, NT3RegenerationGAP43, CGRP

Tubulinp75 apoptosis in Schwann cells

regeneration

Exogenous NTFs reverse effects of injury & aid

regeneration

trkBGAP43

p75

BDNF, GDNF, NT4

***

or Schwann cell derived trophic factors

Regeneration mode

Adult injuries that kill:Repeated nerve injury

EAAtoxicity

SA

Immature state

trkBGFR1

p75NOS

Exogenous

BDNF, GDNF prevents death

***

Regeneration possible GAP43, CGRP, Tubulin

Adult injuries that kill: Avulsion

EAAtoxicity

No transport of Schwann cell derived

NTFsNo access to

Schwann cells

Activation of p75 death pathway?

Rescuing dying motoneurons Preventing excitotoxicity

• Riluzole

• NOS inhibitors

• Glutamate inhibitors MK801

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

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 7:108-115

• Ma J et al. 2001 139: 216-223