Mitochondrial influences in motor disorders

Question 1

What are examples of neurodegenerative disorders with motor symptoms?

Answer 1

• parkinson’s disease
• Alzheimer’s disease
• Huntington’s disease
• Amyotrophic lateral sclerosis
• Charcot-marie-Tooth’s disease
• Friedreich’s ataxia

Question 2

What are the pathologies and symptoms of HD?

Answer 2

striatal lesions caused by the loss neurones
involuntary choreiform movements
behavioural/cognitive impairments- irritable and demented
neuropsychiatric symptoms such as hallucinations

Question 3

What is the life expectancy of HD patients?

Answer 3

premature death - die within 10 years of first symptoms occurring

Question 4

What is the cause of HD?

Answer 4

caused by a polyglutamine expansion - CAG repeat in the huntingtin protein
it is a genetic hereditary disorder

Question 5

What happens to HD patient’s weight?

Answer 5

they show extreme weight loss even though they are eating a normal diet - they become emaciated- this indicates problems with metabolic processes indicating mitochondrial dysfunction

Question 6

What changes have been demonstrated in HD patients?

Answer 6

changes in glucose metabolism in certain areas of the brain
- there is a loss of glucose uptake over time in the striatum (caudate nucleus and thalamus) and also in the cortex - determined by looking at the levels over time using different types of scans
the control patients didn’t show reductions in glucose uptake

Question 7

What was shown about glucose metabolism by covariance patterns?

Answer 7

it showed reduced glucose metabolism in the striatum- caudate nucleus and putamen and increased glucose metabolism in the occipital lobe of HD patients compared to controls

Question 8

What was seen in patients that weren’t yet showing symptoms of HD ?

Answer 8

there were already changes in glucose metabolism in the caudate, putamen and thalamus compared to controls indicating that these changes precede the symptoms of the disorder.
There were no changes in the motor cortex, globus pallidus or cerebellum

Question 9

What does 1H NMR spectroscopy measure?

Answer 9

measures the appearance of molecules in the respiratory chain

Question 10

What was shown by 1H NMR spectroscopy?

Answer 10

decreased N-acetylaspartate
increased lactate suggesting anaerobic respiration
basal ganglia
symptomatic/presymptomatic HD- suggests these changes are occurring before cell death occurs so may be the cause of cell death which then leads onto the symptoms

Question 11

What processes appear to be affect in symptomatic/presymptomatic HD patients?

Answer 11

reduced oxidative phosphorylation 
reduced TCA cycle 
reduced complexes 2, 3 and 4 
reduced aconitase 
NO change in complex 1 activity like PD

Question 12

What did yeast induced to express 103Q show ?

Answer 12

• demonstrated changes in the shape of their mitochondria over time - they look like they aggregate with huntingtin protein
• they demonstrate increased abnormal mitochondrial distribution in yeast overtime
• decreased cell respiration
• altered amount/function of complexes 2 and 3
• altered mitochondrial morphology/distribution

Question 13

What was seen in the yeast expressing 25Q ?

Answer 13

this number of repeats isn’t sufficient to cause HD

there was little overlap between the mitochondria and huntingtin protein

Question 14

What did the HD mtDNA deletion 4977 show ?

Answer 14

There were accumulations of this mitochondrial DNA deletion in the temporal lobes of HD patients
using PCR there was no staining for deleted mutations in the control patients but there was substantial staining for deleted mutations in the HD patients

so there appear to be mtDNA deletions in HD patients

Question 15

What are suggestions that mitochondria are involved in HD?

Answer 15

• mutant huntingtin localised to mitochondria which doesn’t happen normally
• mitochondrial calcium handling is altered
• transcriptional dysregulation in HD

Question 16

What are the early on effects that precede symptoms in HD?

Answer 16

• these are powerful indicators of importance

changes in mitochondrial function which can lead to the production of ROS
changes in respiration which can lead to activation of apoptotic processes

Question 17

What are the symptoms of ALS?

Answer 17

degeneration of both upper and lower motoneurons causing flaccid paralysis of voluntary muscles

Question 18

What are the different forms of ALS?

Answer 18

90% sporadic

10% familial - 20% of these are caused by SOD1 mutations (2% of all cases)

Question 19

What are the other mutations in ALS?

Answer 19

TDP-43 mutations (RNA transactivation response DNA binding protein 43)- it binds to DNA and alters transcription
TDP-43 accumulation (sporadic form)- indicates this binding protein is important in both forms
FUS/TLS mutations (fused in sarcoma/translocated in liposarcoma)

Question 20

What did they do to look at the effects of human TDP-43 in mice?

Answer 20

they overexpressed it in different types of mice

• different mice demonstrated different degrees of expression relative to their endogenous expression of the protein
  1) TAR4/4 - had highest expression
  2) TAR6/6
  3) TAR4
  4) TAR6 - Lowest expression

Question 21

What was seen in the mice models that demonstrated the high levels of expression of human TDP-43?

Answer 21

Demonstrated most drastic changes in limb movements - feet didn’t hang down and toes didn’t splay when held upside down- these effects occur earlier in those that expressed high levels of TDP-43
changes in their gait- smaller steps and they stumbled more
reductions in motor function
reductions in survival
-TAR4/4 all died after 30 days while TAR6/6 were able to live up to about 12 months

Question 22

What was shown in transgenic mice with high levels of TDP43?

Answer 22

Higher levels of ubiquitin aggregates shown by increased ubiquitin staining
ubiquitin aggregates are close to the nucleus of cells
Changes in mitochondria are associated with the ubiquitin

Question 23

What other changes were indicated in the transgenic TDP43 mice?

Answer 23

changes in the levels of mitofusin 1 - important for mitochondrial fusion - mutations in this gene cause changes in the structure of the mitochondria
there are also reduced number of mitochondria in the motor cortex in these mice
24 day old TDP43 transgenic mice have very reduced motor function- one even looked completely paralysed

Question 24

What are the symptoms seen in sporadic ALS?

Answer 24

evidence which suggests mitochondrial dysfunction

• increased CSF levels of 8-hydroxy-2’- deoxyguanosine which is indicative of oxidative damage in the brain
• altered respiratory chain enzymes - these activities tend to be reduced
• CNS energy hypo metabolism in both the spinal cord and motor cortex

Question 25

What changes were seen in SOD1 G93A ALS Tg mice?

Answer 25

• altered mitochondrial morphology
• then decreased mitochondrial respiration due to reduced function
• decreased oxygen consumption
• decreased mitochondrial calcium loading capacity
• decreased respiratory chain complex activities
• decreased ATP synthesis
  ALL this is likely to cause degeneration of nerve cells

Question 26

What changes were seen in SOD1 G37R neuronal cells?

Answer 26

morphological abnormalities of mitochondria
reduced acitvity in complex 1, 2 and 4
reduced mitochondrial membrane potential
decreased levels of cytosolic ATP

Question 27

What happens in SOD1 mutants?

Answer 27

DIRECT mitochondrial interaction
- localisation in the mitochondria inter membrane space, matrix and outer mitochondria membrane - forms macroscopic aggreagtes of SOD1
compartmentalizes in the matrix
it has been suggested to be caused by failures in the folding of the protein but its not clear
damages the mitochondrial membrane so mitochondria can no longer function properly leading to decreased mmp, swelling, vacuole degeneration, impaired calcium and redox homeostasis, increased mitochondrial mediated apoptosis
binds to anti-apoptotic protein Bcl-2, therefore cause apoptosis

Question 28

What are the main changes occurring in ALS?

Answer 28

altered mitochondrial function and induce apoptosis
also altered mitochondrial membrane potential
impairs activity of the respiratory chain leading to increased ROS and loss of calcium homeostasis leading to neurodegeneration