Lecture 18 - Molecular/Cell Biology of Alzheimer's Disease

Question 1

Describe the epidemiology of Alzheimer’s disease.

Answer 1

Predominantly a disorder of the elderly.
10% of over 70s have dementia.
500,000+ people in UK have some form of dementia, 50-60% of all dementia is Alzheimer’s.

Question 2

What are the proteins primarily involved in AD?

Answer 2

Amyloid precursor protein (APP)
Alpha-secretase
Prensenilin (PSEN1 and PSEN2)
Beta amyloid converting enzyme (BACE-1)

Question 3

How long is amyloid-beta found in senile-plaques?

Answer 3

40-42 amino acids long

Question 4

What does the peptide sequence of amyloid beta senile-plaques show?

Answer 4

The peptide comes from a much larger amyloid precursor protein on chromosome 21.

Question 5

Why are people with Down syndrome at higher risk of developing early onset AD?

Answer 5

People with down syndrome have 3 copies of chromosome 21 and therefore 3 copies of APP that gives rise to the amyloid-beta peptide, so there is an increased risk of amyloid-beta senile plaques.

Question 6

Describe the amyloid cascade hypothesis.

Answer 6

All AD pathology can be related to the dysmetabolism of APP and the production of amyloid-beta peptide.
Hypothesis suggest that it is APP and amyloid-beta in some form that initiates and eventually causes AD.

Question 7

Describe the pathology of AD.

Answer 7

Main lesions - senile plaques ad neurofibrillary tangles.
Secondary changes - neuronal loss, loss of synapses, increases in glial activity causing neuroinflammation, pathology of amyloid angiopathy.

Question 8

Describe senile plaques.

Answer 8

Composed of 40-42 amino acid peptide called amyloid-beta.
Extracellular.
Aggregates to form up to 20% of cortical volume (grey matter).
Causes synaptic disruption.
Found around vasculature as cerebral amyloid angiopathy.
20-200um in size.

Question 9

Describe amyloid precursor protein for normal function.

Answer 9

Found on most cells as a type I membrane protein with the N-terminus sticking out of the cell, a short transmembrane domain and a C-terminal tail in the cell.
APP sits in cell membrane with large extracellular domain rejecting out into the extracellular space.
Involved in synapse maintenance, only known function is as a protease (protease nexin II) involved in the regulation of blood clotting.

Question 10

Describe damaging amyloid-beta.

Answer 10

If a mouse is generated to express high levels of APP in the brain it develops amyloid pathology and shows memory deficits.
Damaging amyloid beta domain sits in the membrane.
Cleavage of APP to generate amyloid beta by action of 2 secretase enzymes (beta and gamma secretase).
Aggregation of amyloid beta to form senile plaques and ADDLs.

Question 11

What is the good and bad APP metabolism?

Answer 11

Alpha secretase is not pathogenic and 95% of APP metabolism is via the alpha secretase route normally and does not produce amyloid beta.
Beta and gamma secretase combined give pathogenic metabolism, generating amyloid beta peptide.

Question 12

Describe how alpha secretase metabolises APP.

Answer 12

Several enzymes can cleave APP within the amyloid beta sequence to prevent amyloid beta production.
ADAM10 and TACE do most of the metabolism and operate in the trans Golgi network or at the cell surface.

Question 13

Describe beta and gamma secretase activity.

Answer 13

Beta secretase cuts APP first then gamma secretase.
Stacks form senile plaques.
Action of gamma secretase is in a membrane.
Mutations in APP around the beta-amyloid sequence make APP a better substrate for beta and gamma secretase enzymes.

Question 14

What are ADDLs?

Answer 14

Amyloid beta derived diffusible ligands.
Soluble oligomers of amyloid beta.
Possibly the toxic component from senile plaques.
Represent single or small aggregates of amyloid-beta that interact with nerve cells and cause abnormal nerve cell transmission or directly kill cells.
Target for vaccinations effects.

Question 15

Example and explain anti-amyloid beta therapy.

Answer 15

Immunisation of AD mice models with amyloid beta peptide > reduces senile plaques.
Tried in humans and appeared to work as neuropathology on the patients who came to autopsy showed areas of the brain with reduced or cleared amyloid beta deposits.
However 5% of patients developed aseptic meningitis probably related to amyloid beta deposits in the blood vessels and antibodies reacting to this.
Approach changed to use passive immunisation with engineers humanised monoclonal antibodies. Some success but modest effects on condition so probably best to be immunised when patients only have mild cognitive impairment.

Question 16

What chromosomes are PSEN1 and PSEN2 located on?

Answer 16

PSEN1 = chromosome 14
PSEN2 = chromosome 1

Question 17

What is presenilin, how is this linked to familial dementia?

Answer 17

Presenilin exists in a complex and is the catalytic component essential for gamma secretase activity.
Mutations in PSEN1 and PSEN2 usually cluster around the membrane domain and effectively produce a better gamma secretase.
AD caused by APP mutations are rare but does happen and it called familial dementia.
60% of FD cases have an onset of 30-65 with the average age being 45.

Question 18

Describe presenilin.

Answer 18

Presenilin forms a large 200kDa+ complex with other proteins.
Located in the ER and trans Golgi network.
Knockout reduces amyloid beta and increases beta CTF99 to give a notch phenotype.

Question 19

How do mutations of PSEN1/2 increase gamma secretase activity?

Answer 19

2 parts - 25kDa NTF and 18kDa CTF.
Forms an inter membrane pore and can cleave type I membrane proteins.
These signalling molecules can cause neurons to undergo programmed cell death when cleaved by PSEN.

Question 20

Describe the advancements in PSEN inhibition.

Answer 20

Inflammation thought to be implicated in AD as those with RA have an increased risk.
Patients on anti-inflammatories show reduced levels of AD presumed to be due to COX inhibition.
Standard inflammatories can inhibit PSEN.
LY-411.575, DAPT, BMS-708163 are PSEN inhibitors in clinical trial for AD but not showing good effects on cognition and have several side effects.

Question 21

How is apolipoprotein E associated with AD?

Answer 21

APOE has normal function in the transport of cholesterol in the body and is produced in the brain.
APOE gene located on chromosome 19.
A allele for APOE, E4 is not common but can cause amyloid-beta to stick together rapidly, promoting AD and people that carry one or more APOE E4 genes are at increase risk of AD.
Contributes to familial and sporadic AD.
People with 2 APOE E4 alleles has 20% more pathology than someone with only one allele.

Question 22

Summarise how amyloid-beta senile plaques can be increased.

Answer 22

1. Increasing production of APP - located on chromosome 21 so Down syndrome more susceptible as have 3 copies and some families (rare) have two copies of APP on one chromosome.
2. Mutation in APP around the amyloid-beta sequence causing APP to be a better substrate for beta and gamma secretase instead of alpha.
3. Mutation in PSEN making gamma secretase enzyme activity better and more able to make amyloid-beta.
4. Having APOE E4 making amyloid-beta more likely to aggregate and form senile plaques.
5. Reduce degradation of amyloid-beta by brain enzymes such as IDE and MMP9.
6. Reduce degradation of amyloid-beta by reducing the normal clearance by A2M, APOE and LRP.

Question 23

What are some potential treatments for AD?

Answer 23

Talsaclidine - compound that acts through the muscarinic M1 receptor and increases alpha secretase activity. Shown to reduce amyloid-beta in the CSF of AD patients but there was no improvement on cognition.
Statins, oestrogen and testosterone can all increase alpha secretase activity > potential treatments for AD patients.