Lecture 19 - Molecular Pathology of Alzheimer's Disease Flashcards
Patient in whom Alzheimer’s disease was first described
Auguste D
Estimated number of patients with Alzheimer’s wolrdwide
35.6 million
Estimated number of Alzheimer’s patients in Australia
500,000
Current worldwids cost of Alzheimer’s treatment per year
US$600 billion
Chance of Alzheimer’s after 65
Doubles every 5 years after 65 (1 in 4 chance after 80)
General neuropathology of Alzheimer's 1) 2) 3) 4) 5) 6) 7)
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
Meaning of ‘amyloid’
Starch-like
Amyloid plaque structure
Aggregated amyloid-beta peptide
Forms fibrils
Beta-sheet structure
Stain used to view amyloid plaques
Congo red
Appears as green-red birefringence
What is green-red birefringence?
Amyloid plaques, when stained with congo red stain, under natural light appear red.
Under polarising light appear green
Do amyloid plaques stay in the brain?
Not normally.
Are rapidly turned over
What are amyloid plaques associated with?
Secondary inflammation
Are metal ions present in amyloid plaques?
Yes.
Not clear if they are causative, or just sequestered in plaques
What does ‘amyloidogenic’ mean?
Congo red stain, plaques appear red under normal light, green under polarised light.
Also called birefringence
Fibril protein formation 1) 2) 3) 4) 5)
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
Stages of amyloid plaque formation 1) 2) 3) 4)
1) Primary structure is misformed
2) Monomers
3) Protofibrils
4) Mature fibrils
5) Amyloid plaques
Which part of beta-amyloid is thought to be toxic?
Oligomeric form
Stage between monomeric and fibrillar beta-amyloid
1)
2)
3)
1) Monomers aggregate into oligomers
2) Oligomers can be cross-linked by specific amino-acid modifications that increase stability of monomer
3) Fibrils form
Example of an amino-acid modification leading to beta-amyloid oligomer cross-linking
di-tyrosine cross-link
Number of beta-amyloid monomers in an oligomer
2 to over 10
Amyloid beta structure
1)
2)
1) Hydrophobic
2) 40-42 amino acids in length
How is amyloid beta generated?
1)
2)
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
Where does amyloid-beta deposit?
Brain parenchyma (extracellular space)
Protein from which amyloid-beta is cleft
Amyloid protein precursor
How is amyloid-beta cleaved from amyloid protein precursor?
1)
2)
3)
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.
Where in the brain does amyloid protein precursor often get cleaved by BACE and gamma secretase?
Neuron membranes in grey matter of the brain
Particularly in the cerebral cortex and hippocampus
What is the amyloid protein precursor?
1)
2)
3)
1) An integral membrane protein
2) Concentrated at synaptic termini in the brain
3) Function is unknown, but has many processes associated with it
Processes associated with amyloid protein precursor 1) 2) 3) 4)
1) Growth promotion
2) Regulation of synaptic function
3) Metal homeostasis
4) Cell signalling
Does amyloid protein precursor undergo much post-translational modification?
Yes. Extensively.
Why do people with Downs syndrome have extra amyloid protein precursor and amyloid-beta?
Amyloid precursor protein gene is on chromosome 21
How regularly is CSF amyloid turned over?
~8% turned over every 36 hours
Enzymes that degrade amyloid 1) 2) 3) 4)
1) Insulin degrading enzyme
2) Neprilysin
3) Matrix metalloproteases
4) Angiotensin converting enzyme
Reduction in which enzyme activity has been observed in Alzheimer’s brain?
Protease
Which glial cells can remove amyloid protein?
Microglia.
Can especially remove aggregated protein and plaques
Which difference in formation and degradation of amyloid plaques can lead to accumulation?
As little as a 2% difference
What are neurofibrillary tangles?
1) Protein aggregates found within neurons in Alzheimer’s brain
2) Made of hyperphosphorylated tau
What is tau?
A microtubule protein in neurons
Hyperphosphorylation can lead to neurofibrillary bundle formation
Another name for neurofibrillary tangles
Paired helical filaments
What are tau associated with?
Alzheimer’s
Tauopathies
Normal tau function
Attached to microtubules, allow intracellular transport
How can tau become pathogenic?
1)
2)
3)
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
Which protease cleaves amyloid protein precursor in healthy people?
Alpha secretase
Enzymes that phosphorylate tau
gsk3, cdk5
Suggested neuropathology of Alzheimers
Formation of amyloid-beta oligomers leads to overexpression of gsk3/cdk5, which leads to neurofibrillary tangle formation, as well as plaques
Examples of oxygen radicals
1)
2)
3)
1) Superoxide
2) Peroxynitrite
3) Hydroxyl radical
Sources of oxygen radicals in the brain 1) 2) 3) 4) 5)
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)
Why might neurons be at risk of oxidative stress?
They don’t divide much, so a single cell can accumulate oxidative damage
Consequences of excessive oxygen radicals
1)
2)
3)
1) Lipid peroxidation
2) Protein oxidation
3) DNA oxidation and strand breaks
Downstream effects of oxidative stress
1)
2)
3)
1) Neurotoxic action of altered lipids - apoptosis
2) Accumulation of aggregated protein can disrupt normal protein turnover
3) DNA damage can alter transcription
Biometal contribution to AD 1) 2) 3) 4) 5)
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
How does copper contribute to amyloid-beta oligomer formation?
Copper allows cross-linking of monomers to form stable dimers of amyloid-beta
Metal binding sites on amyloid-beta
Histidine, methionine/tyrosine residues involved in copper/zinc coordination
How might zinc be associated with amyloid plaque formation?
Could displace copper from amyloid-beta binding site, maybe preventing oligomer formation
How can copper lead to free radical formation?
Is reduced by amyloid-beta from copper (II) to copper (I).
This generates oxygen radicals
Why is inflammation initiated in AD?
Response to plaque formation, damaged neurons
Microglia are activated by this
Cells contributing to inflammation in AD
Microglia, monocytes infiltrate
