Lecture 20 - Alzheimer's Disease - Molecular Pathogenesis Flashcards Preview

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Flashcards in Lecture 20 - Alzheimer's Disease - Molecular Pathogenesis Deck (52):

Describe the 'discovery' of Alzheimer's disease

What are the two key pathological features that were observed?

1906: German neuropathologist Alzheimer
• He documented the first case:
• Person called Auguste D

Examination of Auguste D's brain post-mortem revealed:
• Globs of sticky protein between Neurons: Amyloid
• Bundles of fibrils within neurons: Neurofibrillary tangles


Is AD a normal part of ageing?

No, it is a distinct disease


Describe the features of Amyloid plaques

What is amyloid?

Amyloid is a starch-like material

Composed of:
• Aggregated Amyloid-β peptide
• Other components as well:
• Metal ions

• Beta-pleated sheet structures (protofibrils) form fibrils, then form plaques

• Rapidly turned over in the brain
• Associated w/ secondary inflammation


What still remains a mystery in AD?

Why amyloid aggregates & induces neuronal damage only late in life


How many people in Australia w/ AD?

Half a million

This is predicted to increase in coming year
• Economic, social & public health burden


Describe the general neuropathology in AD

• Gross atrophy
• Extracellular neuritic (amyloid) plaques
• Intraneuronal neurofibrillary tangles
• Cerebrovascular amyloid angiopathy
• Activation of microglia & hypertrophy of astrocytes
• Dementia / memory impairment
• Loss of neurons


What is the role of microglia in the brain?
Describe their role in AD

Resident macrophages in the brain

In AD they are:
• Overactive
• Inflammatory response


What is the name for the loss of memory that occurs w/ dementia?
What is responsible for this memory loss?

Amnestic dementia
• More about loss of synapses, rather than loss of neurons


Describe what is thought to be the underlying factor in memory loss in early & late dementia

Early dementia: loss of synapses

Late dementia: loss of the neurons themselves


Describe gross brain atrophy in AD

As the disease progresses, there is loss of neurons, leading to gradual shrinkage of the brain

In severe AD (takes years to get to this stage) the brain is extremely small compared to a normal brain


Compare location of the amyloid plaques & neurofibrillary tangles

Amyloid plaques:
• extracellular; throughout the grey matter
• i.e. in brain parenchyma
• Also within the vasculature of the brain

• Intracellular (within the neurons)


Describe Green-red bifringence
What is this used for?

Identification of 'Amyloidogenic' structures

Congo red stain, then amyloid:
• Appear red in normal light
• Appear green in polarised light

Indicates presence of beta pleated sheet structure within the plaque


Describe how protofibrils can form, what they are, and what the significance of this is

A protofibril is this beta-sheet structure

Under certain environmental conditions the protein structure can partially unfold and form beta pleated sheets

Protofibrils mature into fibrils, then plaques

1. Partially folded protein monomer (A-B peptide)
2. Association of these monomers into protofibrils
3. Fibrils
4. Plaques


What are the oligomers?
Describe their importance in AD

Many of the subunits:
• the Aβ peptide
coming together to form a larger structure

Can become cross linked:
• di-tyrosine cross link
increasing the stability of the oligomer

They are through to be the primary toxic form of Aβ
• Monomers not harmful
• Amyloid fibrils possible just the end point, and not harmful in and of themselves


Describe the generation of Aβ

• Hydrophobic
• Sits in membranes of cells
• Cleaved from a larger precursor protein by 'secretases'
• Cleavage occurs at the membrane
• Released into the extracellular space
• NB can be recycled back into the cell by endocytosis
• Deposited between cells


How long is the Aβ peptide?

40-42 aa

Depends on disease state


What are secretases?

These are proteases

These cleave the Aβ peptide from larger Amyloid precursor proteins


What is APP?

Describe features & function

Amyloid precursor protein

• Integral membrane protein concentrated at synaptic connections
• Gene located on chromosome 21 → implication for people w/ Downs syndrome
• Many different domains
• Many post-translational processing

• Unknown, but many activities associated:
- growth promotion
- regulation of synaptic function
- metal homeostasis
- cell signalling


Describe the cleavage of Aβ from APP

What happens to APP after cleavage?

1. Beta-secretase (BACE) cleaves beta site on the protein
2. Gamma-secretase cleaves gamma site on the protein
3. Aβ released from APP

APP released as soluble form


What is alpha-secretase?

Another secretes that can cleave APP
If this happens, Aβ is prevented from forming


Where does cleavage of APP occur?

At the cell membrane of neurons in the grey matter

In particular in hippocampus and cerebral cortex


Describe the structure of APP

Intracellular domain
Transmembrane domain
Extracellular domain
• Aβ peptide
• BACE cleavage site distally
• gamma-secreatase cleavage site proximally


What is sAPPbeta?

Compare this with sAPPalpha

Part of APP that is cleaved off initially by BACE
Released in soluble form

sAPPapha is the fragment of APP produced when alpha-secretase cleaves in the middle of the Aβ peptide


Where does alpha-secretase cleave?

In the middle of the Aβ peptide


List some general post-translational processes



Describe amyloid turnover

What factors determine accumulation?

Originally: thought to be permanent
Now: we know they are rapidly formed and degraded

Total number increases w/ age

Accumulation depends on the balance between:
• Synthesis
• Degradation

Small changes can affect total accumulation:
• 2% imbalance can result in accumulation
• Very fine balance


Are amyloid plaques in the brain diagnostic for AD?

Not necessarily
There are some people found to have the plaques who do not have AD


What enzymes in the brain can degrade amyloid peptide?

Describe the activity of these enzymes over the course of life

• Neprilysin
• Insulin degrading enzyme
• Matrix metalloproteases
• Angiotensin converting enzyme (ACE)

Reduction in the activity of these proteases as AD progresses
→ more accumulation


Describe the role of microglia in AD

• Inflammatory cell resident in the brain
• Can remove amyloid by phagocytosis


What are thought to be some of the normal functions of amyloid?

Compare this with the abnormal functions

• Antioxidant molecule
• Modulation of metal homeostasis (Cu, Zn)

Abnormal functions:
• Aggregation into amyloid
• Aggregation into oligomers
• Neurotoxic effects
• Inflammation


Describe Neurofibrillary tangles (NFTs)

What are the comprised of?

Describe how they are formed

Composed of:
• Hyper-phosphorylated form of Tau
• Become insoluble and aggregate intracellularly

1. Hyper-phosphorylation of Tau
2. Tau released from microtubules
3. Released tau proteins wrap around each other to form Paired Helical Filaments


What is Tau?

What are the effects on the cell when there are abnormalities?

Normal protein attached to microtubules

When Tau is released, the cytoskeleton cannot perform intracellular transport


What is the effect of oxidative stress on tau?

Further cross linking
Inhibition of degradation of NFTs


What happens to the function of cells as a result of NFTs?

1. Destabilisation of microtubules
2. Accumulation of paired helical fragments
→ Neuronal death & dysfunction

Inhibition of intracellular transport along the microtubules


Describe the conflict between the Baptists and the Tauists

• Amyloid accumulation occurs first
• This is what causes the NFTs

• Tau phosphorylation & NFT formation is the most important feature of AD & occurs first
• Aβ accumulates later

The literature seems to indicate that Aβ deposition occurs first
• Study w/ transgenic mice w/o Tau genes
• Oligomers formed form Aβ peptide influence kinases that lead to tau dysregulation (not the other way around)


What are 'senile plaques'

aka Aβ plaques


List some of the contributing factors in AD

• Age
• Oxidative stress
• Loss of normal neuronal metal homeostasis
• Inflammation

These all play an important role in driving AD


Describe the role of oxidative stress in AD
• Source of free radicals
• Effect of free radicals
• Why neurons are particularly susceptible

• Mainly oxygen, but also nitrogen free radical species

• Mitochondrial ETC: ROS leakage
• Biometals and Aβ plaques
• Inflammation: activated microglia

• Lipid peroxidation, protein oxidation, DNA oxidation
• Neuronal dysfunction and death

• Neurons are permanent cells: build up oxidatively damaged molecules. Cell can't just undergo apoptosis when this happens
• Neurons have low anti-oxidant levels


What is the effect of free radicals on the following:
• Lipids
• Proteins

Describe the down stream effects of these processes

1. Lipid peroxidation
→ Apoptosis

2. Protein oxidation
→ Accumulation of aggregated protein
→ Disruption of axonal transport

3. DNA oxidation & strand breaks
→ Altered DNA transcription


Describe the role of Biometals in AD

What are these biometals?

• Copper
• Iron
• Zinc

• Altered Zn and Cu metabolism in the AD brain
• High levels of Cu and Zn in AD plaques
• High extracellular levels of Cu and Zn in AD brain, especially where neuronal degeneration is highest
• Loss of metals from within the cell


Describe normal biometal homeostasis and how this can be interrupted

• Zn- and Cu-transporters that move the metals into and out of cells

If there is a breakdown in these processes:
• Zn/ Cu accumulates outside cell
• Depleted Zn/Cu inside the cell


What is the role of biometals in free radical formation?

Metals are the main source of free radicals in the brain


Describe the role of biometals in Aβ

Metals bind to the Aβ at several sites
• involve histamine and tyrosine residues

• Promotes aggregation of the peptide
• Zn: Induces formation of neurotoxic oligomers
• Cu: cross linking between Aβ monomers → neurotoxic dimers


What is seen if biometals are removed from amyloid aggregates?

Reduction in aggregation


Which residues in Aβ peptide bind Cu?



Describe the importance of the different Cu valencies in AD

1. Cu(II) associates with Cu binding region in Aβ
2. O2 reduces Cu(II) to Cu(I)
3. H2O2 oxidates back to Cu(II) and OH+ (hydroxyl radical)
4a. OH+ is very toxic and can leads to cell death
4b. OH+ interacts further with the peptide to stimulate aggregation


What is OH+?

The hydroxyl radical
Very damaging to cells


Discuss inflammation in AD

What initiates the inflammation?

What are the consequences of the inflammation?

Not known whether this is a primary event, or something that occurs later on

• Activation of microglia

• Aggregated amyloid recognised as foreign
• Degenerating synapses and neurons triggering inflammation

• Release of neurotoxic cytokines
• Increase free radical production

→ Further damage to neurons and synapses


Which people are most affected by AD?

Those over the age of 65


What is gliosis?

• Activated micrioglia and
• Hypertrophic astrocytes (swollen)


What is seen in the blood vessels of the CNS in AD?

CAA: cerebral amyloid angiopathy:
• Amyloid deposition in the vessels that supply the CNS


What factors lead to promotion of amyloid accumulation?

1. Decreased protease activity
• Neprilysin
• Insulin degrading enzyme
• Matrix metalloproteinases

2. Metals
• Cu
• Zn