lecture 20 Flashcards
Molecular Pathogenesis of Alzeimer's disease I - general background on Alzheimer's disease - amyloid β peptide aggregation and its role in Alzheimer's disease pathogenesis - Tau microtubule protein and its role in Alzheimer's disease pathogenesis - contributing factors to alzheimer's disease - Oxidative stress - contributing factors to Alzheimer's disease pathogenesis - Biometals - contributing factors to Alzheimer's disease pathogenesis - Neuroinflammation
1
Q
When was Alzheimer’s ‘discovered?
A
- 1906 - German neuropathologist Alois Alzheimer studied a patient called Auguste D: wrote up the first documented case of the disease that was to be named after him
- examination of Auguste D’s brain after death by Alzheimer revealed ‘globs’ of sticky protein (amyloid) between neurons and tangled bundles of fibrils in neurons (neurofibrillary tangles)
2
Q
What are milestones in the history of Alzheimer’s disease?
A
- 1910 - 1940: belief that “senile dementia” is a normal part of aging process
- 1960s: AD believed to be a distinct disease (age-related dementia), not just aging
- 1984: Amyloid β peptide purified and sequenced from AD plaques (Colin Masters, University of Melbourne)
- 1992: amyloid cascade hypothesis developed
- 1994: oxidative stress is an important feature of AD
- 1994: role of metals in amyloid plaque formation
- 1998: amyloid oligomers shown to be more important to AD memory impairment than amyloid plaques
- 2000 onwards - development of drugs to target amyloid generation or increase its removal (clinical trials)
- unknown! - why amyloid aggregates and induces neuronal damage and death only late in life
3
Q
Why study this disease?
A
- estimated 35.6 million patients in the world (more than 500,000 in Australia)
- estimate of 114 million patients by 2050 - World population is rapidly aging - will result in dramatic increase in AD patients
- current world wide cost to health care US$604 billion/year
- chance of getting AD doubles every 5 years after 65 (1 in 4 chance of getting AD after 80)
4
Q
What is the general neuropathology of Alzheimer’s disease?
A
- gross atrophy (shrinkage) of the brain (characteristic but not specific to AD)
- extracellular neuritic (amyloid) plaques
- intraneuronal neurofibrillary tangles
- cerebrovascular amyloid (cerebral amyloid angiopathy, CAA)
- activation of microglia, hypertrophy of astrocytes
- degree of dementia/memory impairment (amnestic dementia) in AD correlates with loss of synapses
- loss of neurons as disease progresses
- amyloid plaques appear as dark circular structures throughout the grey matter
- accumulation of amyloid in the vasculature of the brain
5
Q
What are amyloid plaques?
A
- aggregated amyloid β peptide (forming fibrils)
- green-red birefringence with congo red stain
- many non-amyloid β components in plaques
- high concentration of metal ions
- readily turned over in the brain
- may be ‘end point’ of amyloid pathway
- associated with secondary inflammation
6
Q
What is amyloid?
A
- term amyloid first coined by Virchow in mid 19th century (meaning starch or cellulose)
- amyloidogenic means that protein aggregates appearing red microscopically in normal light but green when viewed in polarised light after staining with congo red dye (termed birefringence)
- distinct from amorphous protein aggregates (that have no structure)
- fibrillar nature and β pleated sheet configuration described by electron microscopy in 1959
- form non-bracing fibrils of up to 8nm in diameter
7
Q
How do we get amyloid formation?
A
- formation of fibrils is not specific for a primary protein sequence
- amyloidogenic proteins can begin as unstructured monomers (little alpha-helix or β-sheet structure)
- with increasing concentration or under certain environmental conditions, β-sheet structure increases
- monomers begin to form parallel β-sheet structure (protofibril)
- protofibrils mature into fibrils and form plaques
monomeric –> protofibrils –> mature fibrils –> amyloid plaques
8
Q
What is the amino acid sequence of Aβ monomer?
A
DAEFRHDSGYEVHHQKL
VFFAEDVGSNKGAIIGLM
VGGVVIA
9
Q
How does oligomeric Aβ compare to fibrillar Aβ?
A
- monomeric Aβ peptide can aggregate to form fibrillar amyloid
- monomeric Aβ can form oligomeric species consisting of 2-10+ monomers packed closely together
- oligomers may become cross-linked by a specific amino-acid modifications (e.g. di-tyrosine cross-link) increases stability of oligomer
- oligomers are thought the be the primary toxic form of Aβ
- small solubule oligomers –> in vitro neuronal function –> synaptic damage and neuronal cell death
- extended fibrils also follow that path and:
- -> amyloid –> ? inert storage form
- moves backwards and forwards
10
Q
How is amyloid generated?
A
Generation of amyloid β peptide:
- amyloid β is hydrophobic 40-42 amino acid peptide
- cleaved from a larger Amyloid Precursor Protein by ‘secretases’ (proteases)
- cleavage occurs at the membrane and amyloid β (Aβ) is released into the extracellular space (but can be recycled back into the cell by endocytosis )
- amyloid deposits form between cells (brain parenchyma)
11
Q
Which peptide tends to aggregate more in Alzheimer’s disease?
A
42
40 seems to be more normal
12
Q
How do we get cleavage of Aβ from APP?
A
- cleavage is by beta secretase (B-secretase or VACE) and then gamma secretase (γ-secretase) to release Aβ from APP
- APP can also be cleaved by α-secretase at a different site and this prevents Aβ from forming
- the remaining APP can be released as soluble APP
- cleavage is known to occur on the membrane of neurons in the grey matter of the brain (especially the cerebral cortex and hippocampus)
- the end part of the APP molecule when cut by alpha-secretases seems to have a protective effect
13
Q
Why is there a bit of a problem in the way we understand/research disease processes?
A
- tend to focus on what we can see easily
- easy to see accumulation of amyloid protein
- there may be changes that we can’t see that are even more important e.g. APP intracellular domain
14
Q
What is Amyloid Precursor Protein?
A
- integral membrane protein concentrated at synaptic connections in the brain
- gene is located on chromosome 21. Downs syndrome (extra Chr 21) over produce APP and Aβ
- APP has domains with different functions such as growth factor-like domain, protease inhibitor domain and metal binding domains
- APP undergoes extensive post-translational processing (phosphorylation, glycosylation and cleavage)
- the function of APP is unknown but a lot of activities have been described:
- growth promotion
- regulation of synaptic function
- metal homeostasis
- cell signalling
15
Q
What is amyloid turnover?
A
- origianlly thought that amyloid plaques were long lasting or permanent
- now believed that they are rapidly formed and degraded but the total number increases with age
- amyloid accumulation (plaques or oligomeric forms) depends on synthesis and degradation
- small changes in either can affect the total accumulation
- ~8% of total CSF amyloid is turned over every 36 hours
- proteases in the brain degrade amyloid peptide leading to its clearacne
- insulin degrading enzyme
- neprilysin
- matrix metalloproteases
- angiotensin converting enzyme
- reduction in protease activity has been observed in AD brain
- microglia can also remove amyloid, especially aggregated peptide and plaques
- recent studies suggest that even a 2% difference between production rate and clearance rate in AD patients may result in accumulation
- potentially, only small decreases in production or small increases in degradation could shift the balance in favour of clearing amyloid as a therapeutic treatment
