Lecture 19 - Molecular Pathology of Alzheimer's Disease

Question 1

Patient in whom Alzheimer’s disease was first described

Answer 1

Auguste D

Question 2

Estimated number of patients with Alzheimer’s wolrdwide

Answer 2

35.6 million

Question 3

Estimated number of Alzheimer’s patients in Australia

Answer 3

500,000

Question 4

Current worldwids cost of Alzheimer’s treatment per year

Answer 4

US$600 billion

Question 5

Chance of Alzheimer’s after 65

Answer 5

Doubles every 5 years after 65 (1 in 4 chance after 80)

Question 6

General neuropathology of Alzheimer's
1)
2)
3)
4)
5)
6)
7)

Answer 6

1) Gross shrinkage of the brain
2) Extracellular neuritic (amyloid) plaques
3) Intraneuronal neurofibrillary tangles
4) Cerebrovascular amyloid (CAA=cerebral amyloid angiopathy)
5) Activation of microglia (inflammation), atrophy of astrocytes
6) Dementia/memory impairment (degree correlates with loss of synapses)
7) Neuronal death as disease progresses

Question 7

Meaning of ‘amyloid’

Answer 7

Starch-like

Question 8

Amyloid plaque structure

Answer 8

Aggregated amyloid-beta peptide
Forms fibrils
Beta-sheet structure

Question 9

Stain used to view amyloid plaques

Answer 9

Congo red

Appears as green-red birefringence

Question 10

What is green-red birefringence?

Answer 10

Amyloid plaques, when stained with congo red stain, under natural light appear red.

Under polarising light appear green

Question 11

Do amyloid plaques stay in the brain?

Answer 11

Not normally.

Are rapidly turned over

Question 12

What are amyloid plaques associated with?

Answer 12

Secondary inflammation

Question 13

Are metal ions present in amyloid plaques?

Answer 13

Yes.

Not clear if they are causative, or just sequestered in plaques

Question 14

What does ‘amyloidogenic’ mean?

Answer 14

Congo red stain, plaques appear red under normal light, green under polarised light.

Also called birefringence

Question 15

Fibril protein formation
1)
2)
3)
4)
5)

Answer 15

1) Not specific to protein primary sequence
2) Amyloidogenic proteins can begin as unstructured monomers, as small alpha-helices or beta-sheets
3) Under certain environmental conditions or increasing monomer concentration, beta-sheet structure increases. Can be a result of protein misfolding
4) Increased numbers of misfolded proteins leads to protofibril formation
5) Protofibrils mature to fibrils. Plaque formation

Question 16

Stages of amyloid plaque formation
1)
2)
3)
4)

Answer 16

1) Primary structure is misformed
2) Monomers
3) Protofibrils
4) Mature fibrils
5) Amyloid plaques

Question 17

Which part of beta-amyloid is thought to be toxic?

Answer 17

Oligomeric form

Question 18

Stage between monomeric and fibrillar beta-amyloid
1)
2)
3)

Answer 18

1) Monomers aggregate into oligomers
2) Oligomers can be cross-linked by specific amino-acid modifications that increase stability of monomer
3) Fibrils form

Question 19

Example of an amino-acid modification leading to beta-amyloid oligomer cross-linking

Answer 19

di-tyrosine cross-link

Question 20

Number of beta-amyloid monomers in an oligomer

Answer 20

2 to over 10

Question 21

Amyloid beta structure
1)
2)

Answer 21

1) Hydrophobic

2) 40-42 amino acids in length

Question 22

How is amyloid beta generated?
1)
2)

Answer 22

1) Cleaved from a larger protein (amyloid protein precursor) by proteases
2) Cleavage occurs in cell membrane, and beta-amyloid is released into extracellular space

Question 23

Where does amyloid-beta deposit?

Answer 23

Brain parenchyma (extracellular space)

Question 24

Protein from which amyloid-beta is cleft

Answer 24

Amyloid protein precursor

Question 25

How is amyloid-beta cleaved from amyloid protein precursor?
1)
2)
3)

Answer 25

1) Beta secretase (BACE) cleaves outer part of amyloid protein precursor
2) Gamma secretase cleaves amyloid-beta from the membrane
3) Alpha secretase can also cleave amyloid protein precursor at a different site, which results in amyloid-beta not forming.

Question 26

Where in the brain does amyloid protein precursor often get cleaved by BACE and gamma secretase?

Answer 26

Neuron membranes in grey matter of the brain

Particularly in the cerebral cortex and hippocampus

Question 27

What is the amyloid protein precursor?
1)
2)
3)

Answer 27

1) An integral membrane protein
2) Concentrated at synaptic termini in the brain
3) Function is unknown, but has many processes associated with it

Question 28

Processes associated with amyloid protein precursor
1)
2)
3)
4)

Answer 28

1) Growth promotion
2) Regulation of synaptic function
3) Metal homeostasis
4) Cell signalling

Question 29

Does amyloid protein precursor undergo much post-translational modification?

Answer 29

Yes. Extensively.

Question 30

Why do people with Downs syndrome have extra amyloid protein precursor and amyloid-beta?

Answer 30

Amyloid precursor protein gene is on chromosome 21

Question 31

How regularly is CSF amyloid turned over?

Answer 31

~8% turned over every 36 hours

Question 32

Enzymes that degrade amyloid
1)
2)
3)
4)

Answer 32

1) Insulin degrading enzyme
2) Neprilysin
3) Matrix metalloproteases
4) Angiotensin converting enzyme

Question 33

Reduction in which enzyme activity has been observed in Alzheimer’s brain?

Answer 33

Protease

Question 34

Which glial cells can remove amyloid protein?

Answer 34

Microglia.

Can especially remove aggregated protein and plaques

Question 35

Which difference in formation and degradation of amyloid plaques can lead to accumulation?

Answer 35

As little as a 2% difference

Question 36

What are neurofibrillary tangles?

Answer 36

1) Protein aggregates found within neurons in Alzheimer’s brain
2) Made of hyperphosphorylated tau

Question 37

What is tau?

Answer 37

A microtubule protein in neurons

Hyperphosphorylation can lead to neurofibrillary bundle formation

Question 38

Another name for neurofibrillary tangles

Answer 38

Paired helical filaments

Question 39

What are tau associated with?

Answer 39

Alzheimer’s

Tauopathies

Question 40

Normal tau function

Answer 40

Attached to microtubules, allow intracellular transport

Question 41

How can tau become pathogenic?
1)
2)
3)

Answer 41

1) Hyperphosphorylation causes tau to break microtubules, release form microtubules
2) This leads to abnormal cytoskeleton structure, inhibition of intracellular transport
3) This increases oxidative stress. Can cause cross-linking that inhibits degradation of neurofibrillary tangles by cell

Question 42

Which protease cleaves amyloid protein precursor in healthy people?

Answer 42

Alpha secretase

Question 43

Enzymes that phosphorylate tau

Answer 43

gsk3, cdk5

Question 44

Suggested neuropathology of Alzheimers

Answer 44

Formation of amyloid-beta oligomers leads to overexpression of gsk3/cdk5, which leads to neurofibrillary tangle formation, as well as plaques

Question 45

Examples of oxygen radicals
1)
2)
3)

Answer 45

1) Superoxide
2) Peroxynitrite
3) Hydroxyl radical

Question 46

Sources of oxygen radicals in the brain
1)
2)
3)
4)
5)

Answer 46

1) Mitochondrial e- transport chain
2) Brain has high oxygen consumption, relatively low levels of antioxidants
3) Amyloid-beta can induce radicals
4) Metals (copper, iron) can catalyse radicals
5) Macrophages release radicals (inflammation)

Question 47

Why might neurons be at risk of oxidative stress?

Answer 47

They don’t divide much, so a single cell can accumulate oxidative damage

Question 48

Consequences of excessive oxygen radicals
1)
2)
3)

Answer 48

1) Lipid peroxidation
2) Protein oxidation
3) DNA oxidation and strand breaks

Question 49

Downstream effects of oxidative stress
1)
2)
3)

Answer 49

1) Neurotoxic action of altered lipids - apoptosis
2) Accumulation of aggregated protein can disrupt normal protein turnover
3) DNA damage can alter transcription

Question 50

Biometal contribution to AD
1)
2)
3)
4)
5)

Answer 50

1) Zinc, copper in AD plaques in high concentrations
2) Zinc, copper in highest concentrations in AD brain where damage is highest
3) In AD brain, extracellular biometal concentration higher, intracellular concentration lower than healthy brain
4) Biometals are the greatest contributors to free radicals in the brain
5) Metal binding promotes amyloid formation

Question 51

How does copper contribute to amyloid-beta oligomer formation?

Answer 51

Copper allows cross-linking of monomers to form stable dimers of amyloid-beta

Question 52

Metal binding sites on amyloid-beta

Answer 52

Histidine, methionine/tyrosine residues involved in copper/zinc coordination

Question 53

How might zinc be associated with amyloid plaque formation?

Answer 53

Could displace copper from amyloid-beta binding site, maybe preventing oligomer formation

Question 54

How can copper lead to free radical formation?

Answer 54

Is reduced by amyloid-beta from copper (II) to copper (I).

This generates oxygen radicals

Question 55

Why is inflammation initiated in AD?

Answer 55

Response to plaque formation, damaged neurons

Microglia are activated by this

Question 56

Cells contributing to inflammation in AD

Answer 56

Microglia, monocytes infiltrate