16Neuro - Motor Neuron Disease

Question 1

What are the features of motor neuron disease?

Answer 1

Late onset (typically between 50-65 years)
Progressive and ultimately fatal degeneration of motor neurons in BRAINSTEM, MOTOR CORTEX AND SPINAL CORD = also has effects on other areas of the brain
Symptoms include:
Weakness
Spasticity
Progressive paralysis
Death due to RESPIRATORY FAILURE = no cure

Question 2

What is the incidence of motor neuron disease?

Answer 2

Incidence: 1-2/100 000; more common in men; incidence increases with age

Question 3

What is the different nomenclature for motor neuron disease?

Answer 3

In the USA, MND refers to a spectrum of diseases including Amyotrophic Lateral Sclerosis
In the UK, MND = ALS

Question 4

Who first described motor neuron disease?

Answer 4

Motor neuron disease was first described by Jean-Martin Charcot in 1869

Question 5

What are the typical pathologies affecting those with motor neuron disease?

Answer 5

MND/ALS include a range of pathologies:
Oxidative damage, excitotoxicity = affecting neurons leading to disruption and cell death
Defects in axonal transport
Deficient protein quality control/protein aggregation
Changes in RNA processing

Question 6

What is the link between motor neuron disease and dementia?

Answer 6

Not associated with dementia but 10-15% of MND patients develop Frontotemporal Lobar Dementia (FTLD)
Almost all MND and 50% of FTLD patients show TDP43 CONTAINING PROTEIN AGGREGATES

Question 7

Within how many years of diagnosis does death usually occur?

Answer 7

Death typically occurs within 5 years of diagnosis

Question 8

What is the epidemiology of sporadic Motor neurone disease?

Answer 8

90-95% of cases are SPORADIC = unknown cause
Risk factors may include:
Exercise! = oxidative stress associated with exercise
Possibly free radical related:
Smoking
Pesticides/herbicides exposure
Heavy metals exposure
High glutamate diet - MSG
High fat diet

Question 9

What is the epidemiology of familial Motor neurone disease?

Answer 9

10% of cases are FAMILIAL
Hereditary form of ALS known as familial:
Familial Amyotrophic Lateral Sclerosis (FALS)
Type 1: Age-dependent AUTOSOMAL DOMINANT trait
10-20% of cases of FMND/FALS associated with mutations in SUPEROXIDE DISMUTASE (SOD1)
SOD part of the defense against free radicals
SOD1 is found in the cytosol
Genetic risk factors
Various polymorphisms associated with INCREASED RISK of MND e.g. TDP43
SOD1 and TDP-43 are the most important mutated genes clinically (not much is known about C9orf72)

Question 10

What is the link between motor neuron disease and SOD1?

Answer 10

SOD1 converts superoxide into hydrogen peroxide = is found in cytosol
Superoxide = some of it comes from the mitochondria = monoradicals can combine with other radicals to become more harmful
Mutations in superoxide dismutase (SOD1), an enzyme involved in free radical defense found in families with FMND/FALS
20% of FMND, 2% of total MND
Also a genetic risk factor
Mutations throughout polypeptide
A4V (alanine to valine mutation at codon 4) in 50% of cases, most aggressive
Mutations may reduce function (toxic loss of function) or alter (toxic gain of function) superoxide dismutase activity
Different mutations with different geographical distributions

Question 11

What is the action of SOD1?

Answer 11

SOD1 is a Cu2+ and Zn2+ containing enzyme which removes the superoxide radical
TOXIC GAIN OF FUNCTION probable with FMND/FALS SOD1 mutants
Some SOD1 mutants have more open channel to Cu2+
Entry of H2O2 to Cu2+ to form •OH = more damaging
Looser binding of Cu2+ in Fenton reaction produces •OH radical
Nitronium ion = nitrosylates tyrosine = may prevent tyr phosphorylation = disrupts signalling pathways
Mutant SOD1 more prone to aggregation

Question 12

What is the interaction between SOD1 and Rac1?

Answer 12

Wild-type SOD1 interacts with Rac1 (a signalling G protein) a regulator of NADPH oxidase in neurons (Nox1) and microglia (Nox2)
Mutant SOD1 increases affinity for Rac1 which locks Nox in active superoxide producing form
May be a trigger for the disease
Nox inhibitors improve survival in SOD1 mutant mice

Question 13

Describe the interactions between mSOD1 and the ubiquitin-proteasome system.

Answer 13

mSOD1 aggregates inhibit the proteasome system = aggregates build up in the proteasome
mSOD1 interacts with Derlin 1 (Degradation in Endoplasmic Reticulum Protein 1), a component of the ERAD pathway
Derlin 1 involved in reverse translocation and degradation of ER proteins
mSOD1 mutants inhibit ERAD, therefore accumulation of misfolded proteins
Misfolded protein accumulation activates ASK1 (Apoptosis Signal-Regulating Kinase 1), a kinase involved in apoptosis

Question 14

What is the link between SOD1 and the mitochondria?

Answer 14

mSOD1 associates with outer mitochondrial membrane in neurons
Release of cytochrome C = a pro-apoptotic signal
Leakage of electrons leads to formation of free radicals
mSOD1 causes oxidative stress in astrocyte mitochondria
Damage to neighbouring neurons
mSOD1 impairs mitochondrial calcium buffering = normally mitochondria can sequester calcium ions from the cytosol = this stops leading to higher cytosolic calcium concentrations

Question 15

What other genetic factors have been linked to SOD1?

Answer 15

TDP43 and FUS relocate in cytosol in FMND and SMND = causing aggregation
Relocalised FUS and TDP43 is associated with the appearance of misfolded/aggregated SOD1
Aggregated (wt)SOD1 appears to propagate in a prion-like fashion

Question 16

Why are motor neurons particularly affected in this disease?

Answer 16

If SOD1 activity in FMND/FALS altered in all cell types why are motor neurons damaged?
High levels of SOD1 in motor neurons
Long axonal processes in motor neurons, more neurofilaments
High metabolic load in motor neurons, more ETC activity = more free radicals = more leakage of electrons from the ETC
Low Ca2+ buffering capacity = calcium concentration changes are more pronounced
Glutamate excitotoxicity and effects of free radical damage on calcium homeostasis
Glutamate hypothesis = glutamate is a neurotransmitter

Question 17

How does glutamate cause excitotoxicity in the cell?

Answer 17

Glutamate acts as an extracellular signal (neurotransmitter)
Glutamate receptors (GluR) present on cell surface of motor neurons
When GluR occupied by Glutamate Ca2+ channel opens and Ca2+ enters the cell from the extracellular space
NMDA (N-methyl D-aspartate) receptor (a form of GluR) produces a slow and prolonged calcium response 
AMPA GluR produce a fast and short calcium response
Regulate various processes
Also activate processes than can lead to cell death = change in Ca2+ concentration

Question 18

What happens following glutamate stimulation of glutamate receptors on motor neurons?

Answer 18

Glutamate acts as an extracellular signal (neurotransmitter)
Glutamate receptors (GluR) present on cell surface of motor neurons
When GluR occupied by Glutamate Ca2+ channel opens and Ca2+ enters the cell from the extracellular space
NMDA (N-methyl D-aspartate) receptor (a form of GluR) produces a slow and prolonged calcium response 
AMPA GluR produce a fast and short calcium response
Regulate various processes
Also activate processes than can lead to cell death = change in Ca2+ concentration

Question 19

What effect does prolonged high intracellular calcium have on a cell?

Answer 19

Prolonged high intracellular calcium will activate a calcium dependent protease, calpain
Calpain proteolytically modifies Xanthine dehydrogenase to Xanthine oxidase which converts xanthine into urea, H2O2 and O2•-
Calcium activates nitric oxide synthase which generates NO•
NO• reacts with H2O2 to produce hydroxyl radical, HO• or with O2•- to produce peroxynitrite

Question 20

Why is excitotoxicity a particular problem for cells with impaired ATP production?

Answer 20

This is always a potential threat but becomes a particular danger where ATP production is impaired and Ca(II) buffering capacity is impaired

Question 21

Why is excitotoxicity important in motor neuron disease?

Answer 21

Motor neurons have low levels of receptor GluR subunits (AMPA GluRs) resulting in increased Ca2+ influx in motor neurons
Free radical damage to glutamate re-uptake system of astrocytes (EAAT2) leads to elevated extracellular glutamate and therefore elevated intracellular Ca2+
Death of MNs releases more glutamate to extracellular space
MNs low in calcium buffering proteins even in healthy cells

Question 22

How does mSOD1 and excitotoxicity link together?

Answer 22

Mutant SOD1 can lead to mitochondrial damage resulting in MND but downstream of primary cause
GluRs/excitotoxicity may “sensitise” motor neurons to primary insult

Question 23

What were the results of experiments carried out on transgenic mice?

Answer 23

Mice with normal human SOD or mutant SOD with increased activity, developed MND symptoms rapidly (increased SOD activity)
Mice expressing mutant human SOD, with decreased SOD activity, develop MND, but more slowly
G39A, G37R mutations lead to vacuolation, inclusions (SOD1, ubiquitin and neurofilaments)
Nitrotyrosine detected in neurofilaments, also increased protein carbonyls and lipid peroxidation
Vitamin E and Riluzole (GluR antagonist) slow progression

Question 24

What role does mSOD1 play in aggregation?

Answer 24

Mutant SOD may interact with neurofilament proteins and release Zn2+ = transition metal = Fenton reaction = hydroxide radicals
Site-specific free radical damage leads to NF aggregates
Disrupt cell architecture and axonal transport
Mutant SOD more prone to aggregation

Question 25

What are the effects of aggregates?

Answer 25

Aggregates impair ubiquitin-proteasome system

Aggregates may propagate like prions

Question 26

What are some of the features of inclusion bodies?

Answer 26

Inclusion bodies often include ubiquitin as well as aggregated proteins
Both SMND and FMND have nitrotyrosine build up

Intracellular inclusion bodies found in MND containing various different proteins, varies with MND type:
Neurofilament proteins (NFL), SOD1, peripherin, 14-3-3 proteins, TDP43 (fragments), ubiquitin
Mutations in TDP43 linked to MND/ALS and frontotemporalobar degeneration (FTDL-U)
wtTDP43 found in aggregates in ALS

Question 27

How is TDP-43 linked to familial motor neuron disease?

Answer 27

Around 4% of FMND cases due to mutations in TARDBP gene for TDP-43
Autosomal dominant inheritance
Mutations segregate with disease
TDP43 in inclusions hyperphosphorylated, ubiquitylated and includes 25kDa C-terminal fragments lacking NLS (nuclear localisation signal)
Accumulates in aggregates in cytoplasm in MND/ALS
“Stress” (oxidative?) may promote aggregation/cytoplasmic localisation

Question 28

What are the main features of TDP-43?

Answer 28

TDP-43 is 414 residues long
Widely expressed, most nuclear
Two RNA recognition motifs and a glycine rich C-terminal domain involved in protein-protein interactions
Mostly missense mutations in C-terminus linked to MND

Question 29

How are FUS/TLS linked to motor neuron disease?

Answer 29

Mutations in Fused in Sarcoma (FUS)/Translocated in Liposarcoma (TLS) associated with ~4% of FMND
Most normally nuclear
Present in cytoplasmic inclusions in FUS FMND, not in inclusions in TDP-43 FMND
Appears to be an RNA binding protein

Question 30

How is TDP-43 linked to FUS?

Answer 30

May regulate transcription of certain genes
Seem to be involved in RNA processing and transport
Shuttle in and out of the nucleus in granules containing RNA in neurons
Appear to be involved in neuronal plasticity
Toxic loss or gain of function?
mTDP43/mislocalised TDP43 may promote wtSOD1 aggregation

Question 31

How is alsin linked to motor neuron disease?

Answer 31

Mutations in alsin gene lead to juvenile onset MND (FALS) - Type 2 FALS
Guanine nucleotide exchange factor for small GTPases e.g. Rab5, Rac1
Involved in membrane changes, pinocytosis (cell-drinking), vesicle trafficking (axonal transport)
Possibly involved in regulation of glial endosome Nox via Rac1

Question 32

What other mutations have been linked to motor neuron disease?

Answer 32

Recently identified mutations in ALS4 (senataxin, SETX) gene lead to juvenile onset FALS
Senataxin appears to contain RNA processing domain
Mutation in VAPB (ALS8) associated with atypical ALS
Involved in vesicle trafficking/fusion and ERAD

Question 33

What are believed to be the causes of sporadic motor neuron disease?

Answer 33

Evidence for free radical damage
Nitro-tyrosine, protein carbonyls
Protein aggregation
TDP43/ubiquitin inclusions
Intracellular aggregates of NOS and SOD in some cases

Excitotoxicity
Reduced neuronal glutamate observed due to increased release/reduced re-uptake?
Changes (compensatory?) in glutathione peroxidase, SOD2 etc.

Question 34

What evidence is there to suggest that environmental toxins may play a part in the development of motor neuron disease?

Answer 34

Environmental toxins
Guam ALS - Chamorro people eat fruit bats which eat cycad fruit - develop MND-like disease
Cycad roots contain excitotoxin produced by bacteria - β-methylamino-L-alanine (BMAA)
From bacteria to bats concentrates BMAA 100,000x
Unknown toxins cause of sporadic ALS?

Question 35

What treatments are available for motor neuron disease?

Answer 35

Anti-oxidants
Vitamin E = no evidence of benefit in human subjects
Co-Enzyme Q10 = no benefit in MND/ALS
Apocyanin = Nox inhibitor, not clinically tested
Riluzole - glutamate receptor antagonist
Some delay in progression (few months)
Neurotrophic factors? = protecting neurons
GM6
Myotrophin (IGF)
Amplifiers of heat shock protein
Arimoclomol
Anti-inflammatory drugs? = link to neuroinflammation