BS42023 L6

Question 1

what are the three classifications of ALS?

Answer 1

• Sporadic; most common form of ALS (90-95% of all cases). These occur randomly with no known cause.
• Familial; inherited disease- accounts for a small number of cases (5-10%).
• Guamanian; an extremely high incidence of ALS was observed in Guam and the Trust Territories of the pacific in the 1950s.

Question 2

what are the clinical symptoms of onset of ALS?

Answer 2

Limbs; weakness of grip, decreased dexterity, foot drop, leg stiffness and tripping

Throat; slurred speech, difficulty chewing or swallowing

Question 3

what are the clinical symptoms of progression of ALS?

Answer 3

Limbs; unable to hold objects, write, feed, or toilet. Unable to walk, stand, or turn over in bed

Throat; unable to speak, swallow food or saliva

Breathing; breathless with exertion or lying flat

Cognition; dementia is rare but subtle deficits are common

Question 4

what causes death of ALS and after how long?

Answer 4

respiratory failure within an average of 22 months from diagnosis

Question 5

The fact that the causes for most cases of ALS are unknown and the clinical course is highly variable suggests what?

Answer 5

multiple factors underlie the disease

Question 6

what do upper motor neurons do and where are they found?

Answer 6

located in the motor cortex and travel down the spinal cord to connect at different levels of cells known as lower motor neurons (LMN).

Question 7

what do lower motor neurons do and where are they found?

Answer 7

LMN travel out of the spinal cord via ventral horn (ie. Along the arms and legs) and connect to muscle.

Question 8

If MNs are damaged or die the main consequence is?

Answer 8

difficulty with voluntary movements

Question 9

what is the pathology of ALS?

Answer 9

Degeneration of the upper and lower motor neurons in the motor cortex, brain stem and the spinal cord.

Question 10

Why do people with MND develop paralysis? (3)

Answer 10

• The pyramidal motor neurons in the frontal lobe degenerate and die causing severe spasticity and mild weakness of muscle groups.
• Motor neurons in the spinal cord degenerate and die causing wasting and major weakness of muscle groups.
• The degenerative process spreads until it affects almost all motor neurons eventually resulting in complete muscular paralysis.

Question 11

How does gene testing help MND patients?

Answer 11

Gene testing can go some way to answering why the disease occurred.

Excluding the presence of gene mutations can be greatly reassuring. IVF and gene testing of defective genes can be prevented from recurring in future generations.

Question 12

Why do we focus on familial MND genes?

Answer 12

Population effect of FMND is small but the biological impact of FMND gene is huge. Only one in ten people with MND have family history of MND.

Question 13

why do motor neurons degenerate?

Answer 13

Normally, TDP-43 resides in the nucleus where it processes gene transcripts.

In ALS, TDP-43 aberrantly accumulates in the cytoplasm (changes location) of motor neurons where it forms aggregates.

Question 14

chick spinal neurons expressing mutant TDP-43 display what? (3)

Answer 14

• Cytoplasmic aggregates
• A reduction in the axonal length
• Cellular toxicity.

Question 15

what animal TDP-43 models have been used to study MND disease mechanisms?

Answer 15

zebrafish, drosophila, cleveland mouse

Question 16

what percentage of ALS is familial?

Answer 16

~5-10%

Question 17

what is the most common cause of familial ALS?

Answer 17

mutations in SOD1 protein

Question 18

what does SOD1 do?

Answer 18

converts superoxide radicals (toxic product of mitochondrial oxidative phosphorylation) to hydrogen peroxide and oxygen.
aka. is involved in antioxidant defence mechanisms.

Question 19

where is SOD1 found?

Answer 19

in cell cytosol, nucleus and mitochondrial membranes

Question 20

What is the role of mutated SOD1 in ALS?

Answer 20

Early studies of mutant SOD1 indicated that disease is not due to loss of enzyme activity. Mice engineered to completely lack SOD1 do not develop ALS.

Mutations must cause SOD1 to gain a toxic property (ie. Gain of function mutation).

Question 21

Potential cellular mechanisms underlying ALS include; (8)

Answer 21

• oxidative damage
• Accumulation of intracellular aggregates
• Mitochondrial dysfunction
• Glutamate excitotoxicity
• Growth factor deficiency
• Glial cell pathology
• Defective axonal transport
• Ca2+ dysregulation

Question 22

what is the role of oxidative damage in ALS?

Answer 22

• Mutations in SOD1 causes structural changes in SOD1 that expose the Cu site to aberrant substrates.
• SOD1 mutations catalyse Cu-mediated breakdown of H2O2 to hydroxyl radicals –> oxidative damage.
• Peroxinitrite is also a possible substrate –> nitration of tyrosine residues in target proteins to kill cells.
• H2O2 and peroxinitrite-mediated oxidative damage both involved Cu.
• Thus, Cu-mediated oxidative damage may play a role in ALS pathology.

Question 23

Mutant SOD1 directly disrupts what?

Answer 23

mitochondrial function and cytochrome c association with inner mitochondrial membrane (interferes with respiration)

Question 24

In ALS, glutamate transport is markedly reduced in affected brain regions due to what?

Answer 24

profound loss of EAAT2

Question 25

Why is there selective degeneration of MNs in ALS?

Answer 25

Only MNs degenerate in familial ALS despite expression of mutant SOD1 in every cell. One possibility is that MNs are more susceptible to excitotoxicity

Question 26

why might spinal MNs be susceptible to excitotoxicity?

Answer 26

• Receive very strong glutamatergic inputs
• Express Ca2+-permeable AMPA receptors
• Have low Ca2+ buffering capacity.
  These properties are crucial for normal MN function but under pathological conditions this could promote MN death.

Question 27

key features of ALS include

Answer 27

1. SOD1 mutation

2. loss of glial glutamate transporter

Question 28

name novel therapeutic targets for ALS (7)

Answer 28

1. Glutamate transport (e.g. Ceftriaxone)
2. protein misfolding and aggregation (e.g. Arimoclomol)
3. mitochondrial targets (e.g. Olesoxime, Dexpramipexole)
4. stem cell therapy (mouse embryonic stem cells treated with retinoic acid and sonic hedgehog)
5. Muscle targets (e.g. ACE-031, CK-2017357 )
6. RNA targets (e.g. ISIS 33361)
7. VEGF (vascular endothelial growth factor) (e.g. increased VEGF)

Question 29

MNs express high numbers of Ca2+ permeable AMPARs making MNs very vulnerable to excitotoxicity. How is this affected in ALS?

Answer 29

• Glutamate stimulates Ca2+ permeable AMPARs.
• This increases intracellular Ca2+ and due to low Ca2+ buffering, some of the Ca2+ is taken up by mitochondria.
• Astrocytic glutamate transporter removes glutamate from synapses.
• Factors released from astrocytes increase GluR2 expression in nearby MNs.
• The presence of mutant SOD1 in neurons and astrocytes interferes with this process.
• Balance is shifted from normal glutamatergic communication between neurons to excitotoxic MN death.