Alzheimer's Disease

Question 1

What are the characterisations of AD?

Answer 1

Progressive loss of short-term memory

Aphasia - loss of speech, poor word recognition

Apraxia - inability to make voluntary movements

Agnosia - poor object recognition

(often also heightened aggression, agitation, sleep distrubances)

Question 2

What are some non-modifiable risk factors for AD?

Answer 2

Age
= 1/6 over 80

Sex
= higher in women

Genetics (familial AD)
= mutations (mostly sporadic) in genes coding for:
APP (amyloid precursor protein)
PS1 (presenilin 1)
PS2 (presenilin 2)

Question 3

What are some modifiable risk factors for AD?

Answer 3

Metabolic / Vascular Factors
= diabetes, hypercholesterolaemia, obesity

Diet / Nutrition
= high saturated fats , low folic acid, vitamins B6 / B12

Lifestyle
= smoking, alcohol, physically inactive
= socially active, mental stimulation can help

Question 4

Summarise the main pathology in AD?

Answer 4

β-amyloid plaques (extracellular)

Neurofibrillary tau tangles (intracellular)

= also:
reactive astrocytes
activated microglia
synaptic dysfunction + neuronal death = brain shrinkage

Question 5

What is β-amyloid

Answer 5

= makes up extracellular senile plaques

= caused by protein misfolding
= found in many diseases in many tissues / organs
= made from approx. 30 different proteins
= identified by congo red / thioflavin stains
= 8-10 nm beta-sheet fibrils
= resistant to proteolysis

= has two major forms:
Aβ40
Aβ42
= spontaneously aggregate in vitro into amyloid fibrils (Aβ42 - more quickly)

Question 6

What are the 2 pathways for processing amyloid precursor protein (APP)?

Answer 6

Non-amyloidogenic pathway

Amyloidogenic Pathway

Question 7

How does the non-amyloidogenic pathway for APP processing work?

Answer 7

APP
enzyme = α-secretase
= sAPPα (soluble)
+
= C83
enzyme = γ-secretase
= p3 = <38 a.a

= NO AGGREGATION

Question 8

How does the amyloidogenic pathway for APP processing work?

Answer 8

APP
enzyme = β-secretase
= sAPPβ (soluble)
+
= C99
enzyme = γ-secretase
= Aβ = 40-42 a.a.

= AGGREGATION

Question 9

What is the toxic oligomer hypothesis?

Answer 9

Process:
Monomers - Oligomers - Protofibrils - Amyloid fibrils

(can jump steps)
(can go both ways)

= oligomers may be the most toxic form of Aβ

Question 10

Name some key APP mutations?

Answer 10

They are named after the countries they were discovered in:

London mutations
Iowa mutaion
Flemish mutation
Swedish double mutation

(over 50 identified)

Question 11

What are neurofibrillary tangles (NFTs)?

Answer 11

process:
microtubule - tau hyper-phosphorylation - paired helical and straight filaments - tau neurofibrillary tangles

= NFTs made from paired helical filaments (two 10-20 nm filaments wound round each other - cross-over at every 65-80 nm)

= paired helical filaments (PHFs) arise from hyperphosphorylation of microtubule-associated protein TAU (MAPT)

= MAPT involved in axonal microtubule assembly and stability

Question 12

What about Tau isoforms?

Answer 12

There are 6 main isoforms of tau

= derived by alternative mRNA splicing from single MAPT gene on chromosome 17

(1 of which is foetal tau)

Question 13

What happens in Tau phosphorylation?

Answer 13

PHFs (paired helical filaments)
= contain all 6 tau isoforms
= tau in PHF is highly phosphorylation (hyper) at multiple Ser-Pro, Thr-Pro site

BUT role of phosphorylation in NFT formation is unclear

Foetal brain = 20% tau phosphorylated
Normal adult brain = <5% tau phosphorylation
PHFs = 100% tau

Question 14

What are MAPT (microtubule-associated protein tau) mutations?

Answer 14

= give rise to frontotemporal dementia (not AD)

= all 6 tau isoforms affected by mutation in:
exon 9, exon 12, exon 13

= only affect tau isoforms with four microtubule-binding repeats
exon 10

= pathogenic mutations alter tau production + lead changes in microtubule assembly or aggregation properties

Question 15

Chronic inflammation in AD?

Answer 15

= a feed forward cycle

Aβ + Tau = cause injury = astrocyte and microglial activation

= increased proinflammatory elements
(e.g. IL-1β, IL-6, TNF-α)

= neuronal damage / death

Question 16

Vascular damage in AD?

Answer 16

Cerebral amyloid angiopathy
= deposition of Aβ in walls of brain blood vessels

Damage to BBB

Dysregulated cerebral blood flow

Changes in cerebral perfusion may be one of earliest features in AD?

Question 17

What are 3 of the hypotheses for AD?

Answer 17

Amyloid Cascade Hypothesis

Tau hypothesis

Cholinergic hypothesis

(EXTRA READING - MORE HYPOTHESES?)
= also INFLAMMATION hypothesis
(chronic inflammation - damage neurons and leads to accumulation of amyloid and tau)

= also OXIDATIVE STRESS hypothesis
(imbalance between production of ROS and detoxifying of them = accumulation leads to neuron damage)

= also VASCULAR hypothesis
(due to reduced blood supply , leaky blood vessels = reduced oxygen + nutrients to neurons)

Question 18

What is the Amyloid Cascade Hypothesis? (+evidence)

Answer 18

= aggreagation of Aβ sets of series of downstream events (synaptic dysfunction, inflammation)

= leads to development of PHFs, NFTs = cell death

Evidence:
= strong association between mutations in genes that increase Aβ production + risk of AD

= treatment with Aβ causes death of neurons

= Aβ can induce tau phosphorylation in vitro

= animals that overexpress human APP mutations develop cognitive impairment (decreased spatial memory)

= treatment of animal models with compounds that prevent Aβ accumulation / remove aggregated Aβ = improve cognitive performance

(EXTRA READING - MORE EVIDENCE)
= genetic mutations in PS1/2 identified as familial AD cause
= imaging studies - can visualise amyloid deposition

Question 19

What is the Tau Hypothesis? (+evidence)

Answer 19

= that tau pathology precedes Aβ PLAQUES
= that NFTs are main cause of neuronal dysfunction and death

Evidence:
= animals that express human tau NFTs develop cognitive impairment + neuronal death

= treatment of animal models with compounds that prevent tau aggregation show improved cognitive performance

= pathological studies have reported tau lesions earlier than Aβ accumulation in human brains

= Braak staging of tau pathology strongly correlated with extent of cognitive + clinical symptoms

= dementia observed in other neurodegenerative diseases where NFTs are present

Question 20

What is the Cholinergic hypothesis?

Answer 20

(EXTRA READING)
= that detrioration of cholinergic neurons is significant factor in development of AD

= acetycholine is important for memory + attention

= cholinergic neurons die early in AD

(treatment - keep Ach in synapse for longer = improved memory)

Question 21

What are the some current treatments for AD?

Answer 21

AChE inhibitors
= donepezil, rivastigmine, glantamine

NMDA (glutamate) uncompetitive antagonist
= Memantine = competes with Mg2+ for channel binding

Combination regiments
= memantine + donepezil

= currently approved treatments are symptomatic

EXTRA READING

Question 22

What are the key drug targets?

Answer 22

Prevent Aβ formation

Prevent, reverse or remove Aβ aggregation

Prevent or remove PHF and NFTs

Question 23

How does immunotherapy (vaccination) work?

Answer 23

= Injection of pre-aggregated Aβ42 into blood-stream causes development of antibodies to fibrillar Aβ

= Antibodies cross BBB , attack senile plaques
(removed by Fc-receptor mediated phagocytosis, disaggregation of Aβ + drainage of soluble Aβ into plasma)

= Antibodies may also block Aβ fibrillation and toxicity

BUT trials stopped due to brain inflammation / death

= passive immunisation trials still going (monoclonal antibodies) - none succeeded

Question 24

How do β-secretase inhibitors work?

Answer 24

= development difficult due to large binding site and need for BBB penetration

= e.g. Lanabecestat, Verubecestat - recently dropped due to lack of efficacy

= worsening of cognitive decline / other side-effects due to interference with BACE progressing of numerous substrates

(involved in ion channel formation, synaptic maintenance, axon guidance, myelination)

(β-secretase = 1st enzyme in amyloidogenic pathway = aggregation of Aβ)

Question 25

How do γ-secretase inhibitors work?

Answer 25

= these inhibitors initially inhibited Notch processing (dysregulated in many cancers) = side-effects

= later ‘notch sparing’ γ-secretase inhibitors

= γ-secretase critical in processing of other type I membrane proteins
(e.g. ErbB4, E-cadherin, N-cadherin, ephrin-B2 , CD44)

= inhibitors dropped due to lack of potency, low brain penetration, poor selectivity, side-effects, lack of efficacy

(β-secretase = 2nd enzyme in amyloidogenic pathway = aggregation of Aβ)

Question 26

What are some other approaches to treatment options?

Answer 26

Metabolism
= repurpose diabetes drugs (e.g. metformin)

Inflammation
= reduce gliosis

Vascular
= improve blood flow

Neurogenesis
= strengthen synapses or regrow neurons

Epigenetics
= modify gene expression