Alzheimer's Disease Flashcards
What is dementia?
Chronic/ persistent disorder of the mental processes caused by brain disease or injury and marked by memory disorders, personality changes and impaired reasoning.
What is used to identify if someone has Dementia?
and how does it work/ identify dementia?
Diagnostic and statistical manual of mental disorders (5th ed).
DSM-5 replaces the word “dementia” with “major cognitive disorder”
It states that symptoms include a decline in memory and a decline in AT LEAST one of the following:
1) Ability to generate coherent speech and/or understand spoken or written language
2) Ability to execute or carry out motor activities
3) Ability to recognise or identify objects, persons, sounds, shapes or smells
4) Ability to think abstractly, make sound judgements and plain and execute complex tasks
Alzheimer’s Association still use the term dementia.
Decline in cognitive ability must be severe enough to interfere with daily life.
What are the most common types of dementia?
- Alzheimers Disease- 62%
- Vascular dementia (multi infarct dementia)
- Dementia with Lewy bodies
- Frontotemporal dementia (inc. Pick’s disease)
- Mixed dementia
- Parkinsons disease dementia
- Creutzfeldt-Jakob disease
- Normal pressure hydrocephalus
- Korsakoff’s syndrome
- Other dementias
State the symptoms of Alzheimers
- Memory loss, important dates, repeated questioning
- Challenges in planning or problem solving
- Difficulty completing familiar tasks, games, locations
- Confusion with time or space (what day is it?)
- Difficulty understanding visual images
- Problems with words spoken/written
- Misplacing things/retracing steps
- Decreased or poor judgment
- Withdrawal from activities
- Changes in mood/personality
State what normal aging is like
- Sometimes forgetting names or appointments
- Making occasional errors
- Occasionally needing help with settings (TV etc)
- Getting confused which day of week but figuring it out
- Vision changes related to cataracts
- Sometimes having trouble finding the right word
- Misplacing things from time to time/retracing them
- Bad decision once in a while
- Feeling weary of work/family/socializing
- Becoming irritable when routine disrupted
Provide key facts about Alzheimers Disease
The Good
* Identified over 100 years ago by Alois Alzheimer and Oskar Fischer
* Research intensified over the last 30 years
* Great amount of knowledge about Alzheimer’s disease gained
The Bad
* Physiological changes that trigger Alzheimer’s disease are unknown
* Rare, inherited forms of disease caused by known genetic
mutations
* No treatment currently exists to slow or stop Alzheimer’s disease
The Ugly
* If you live long enough you will most likely develop Alzheimer’s
disease
* Alzheimer’s disease is fatal
* No strategy in place to deal with dementia
Define prevalence, incidence and life time risk.
- Prevalence: The number of existing cases of a disease in a population at a given time
- Incidence: The number of new cases of a disease in a given time period (1 year)
- Lifetime risk: The probability that someone of a given age develops a condition during their remaining life span. For 65 year old 1 in 5 for women, 1 in 10 for men.
How many people are estimated to be living with dementia in the UK?
850,000
how many people born in the UK this year will develop dementia in their lifetime
1 in 3
Alzheimers is the leading cause of death (Sept 2020)
11.2% in England
11.1% in Wales
Population-Attributable Fraction (PAF).
List potentially modifiable fraction.
Percentage reduction in new cases over a given, if a given risk factor was completely eliminated.
- Education attainment
- Midlife hypertension
- Midlife obesity
- Hearing Loss
- Diabetes
- Physical inactivity
- Depression
- Social isolation
- Smoking
After adjustment for association between risk factors
PAR falls to 35%
Clinical diagnosis of Alzheimers Disease.
National Institute of Neurological and Communicative Disorders/ Stroke & Alzheimer’s Disease and related Disorders Association (NINCDS/ADRDA).
Clinical criteria is used to identify patients that are likely to have developed AD
Sensitivity ~80% (ability to detect patients that have AD)
Specificity ~70% (differentiates between people with/without AD)
Criteria detects short term memory loss, difficulties with daily living & changes in personality
- Historical information from family/friends (timing & severity of symptoms, e.g. gradual onset suggests AD, sudden onset suggests vascular dementia)
- Physical examination (head injury, circulatory problems, stroke)
- Mental state examination
Name the 5 different cognitive assessments
- Mini-mental state examination (Folstein et al., 1975 J. Psych. Res.)
- Blessed Scale (Blessed et al., 1968 Brit. J. Psych.)
- ADAS-cog (Alzheimer’s disease assessment scale – cognitive)
- ADAS-noncog Alzheimer’s disease assessment scale –
(neuropsychomotor) - ADL (activities of daily living)
How is AD diagnosed?
If memory is impaired and two or more deficits in the following tests:
1) Cognitive function- questions e.g. what is todays date? where are we? 7 x table. spell ‘world’ backwards etc.
2) Global measures- measures memory, orientation, judgement, problem solving, community affairs, home and hobbies, personal care
3) Psycopathology- measures depression, behavioural disturbances, anxiety, irritability, comprised of observations from both clinician and informant
4) Functional ability- measures basic activities of daily living (e.g. feeding, eating, toilet) and more complex living activities (shopping, travelling, finances)
Name the different types of Physical and Neurological Assessment that can be done for AD patients?
- MRI (Magnetic Resonance Imaging)
- PET (Positron Emission Tomography)
Check slides 14 and onwards for images
What can an MRI show in patients with AD?
MRI provides detailed high resolution images (1mm)
Gross anatomical changes in ventricles/ sulci can be seen on brain scans.
All occur but not useful, as occur with many different causes of dementia, so no differential diagnostic info obtained. Can rule out tumours and stroke as cause of cognitive decline.
Check slides 14 for images
What can a PET show in patients with AD?
Functional neuroimaging detects metabolically active cells/ brain regions and cerebral blood flow (when the patient has been injected with 18^F-FDG or inhaled 15^O2)
Unfortunately medium resolution images (4mm) involve the use of radioisotopes. They can also be expensive and time consuming- therefore they’re mainly used for research.
Can distinguish AD from FTS; AD shows inactivity in the rear brain. Whereas with FTD the frontal part of the brain is inactive.
Newer agents may be developed (e.g. 18^F-florbetapir) that specifically bind to plaques.
Check slides 15 for images
State the diagnostic & guidelines for AD
Based on clinical judgement about the cause of a patients symptoms
Based on reports from the patient, family and friends
Results of cognitive testing and neurological assessment
Distinguished 3 stages of Alzheimer’s disease
1. Mild/ early stage
2. Moderate/ mid stage
3. Severe/ late stage
Describes Mild/ Early Stage Alzheimer’s
Memory loss is the first sign of AD however:
1. They appear healthy, have trouble making sense of surroundings
2. Commonly mistaken for normal ageing
3. Underlying pathology can occur up to 20 years before symptoms
Describes Moderate/ Mid Stage Alzheimer’s
AD spreads through the brain. The cerebral cortex begins to shrink due to death of neurons
Mild AD signs can include memory loss, confusion, trouble handling money, poor judgement, mood changes and increased anxiety
Moderate AD signs can include increased memory loss and confusion, problems recognising people, difficulty with language and thoughts, restlessness, agitation, wandering, and repetitive statements
Describes Late/ Severe Stage Alzheimer’s
Extreme shrinkage occurs in the brain.
Patients become completely depended on others for care
Hospices/ Palliative care may be needed.
Symptoms include:
weight loss, seizures, skin infections, groaning, moaning, or grunting, increased sleeping, loss of bladder and bowel control
Cause of death usually: aspiration, pneumonia or other infections
What is the new diagnostic criteria and guidelines for Alzheimer’s Disease and what does it propose?
- 2011 National Institute on Ageing (NIS) recommend new diagnostic criteria
and guidelines for Alzheimer’s disease - New guidelines update, refine and broaden guidelines published in 1984
- Incorporate scientific advances in the last 3 decades
Three stages of Alzheimer’s Disease proposed
1. Preclinical Alzheimer’s disease
Measureable changes in biomarkers
2. Mild cognitive impairment (MCI) due to Alzheimer’s disease
Mild but measurable changes in thinking ability, but not enough to
impact on daily life
3. Dementia due to Alzheimer’s disease
Encompasses all stages of AD as previously described
Gross Anatomy- what is the difference between a normal and alzheimers brain?
- cerebral atrophy predominantly in the temporal and frontal lobes
- narrowed gyri
- widened sulci
- cortical atropy leads to compensatory dilation of ventricles
Name the 3 main researchers who looked into Alzheimers?
- Dr Alois Alzheimer
- Dr Auguste Deter
- Dr Oskar Fischer
What are Bielschowsky Stains used for?
- Beta-Amyloid Plaques
Insoluble aggregates of beta-amyloid proteins that form outside
neurons - Neurofibrillary Tangles
Insoluble aggreates of hyper-phosphorylated Tau protein that
form inside neurons
- Symptoms not sufficient for diagnosis
- Require presence and abundance of identifiable morphological
substrates - Plaques and Tangles
What are Tau Protein?
Predominantly expressed in the CNS
In neurons, predominantly found in axons
Co-purifies with tubulin/MT’s, stabilizes MT integrity, promotes MT assembly;
Microtubules are responsible for transport of molecules
(eg neurotransmitter receptors) within neurons
What are Amyloid Precursor Protein (APP)
Function: unknown? Maybe cell adhesion, neurite outgrowth, synaptogenesis, cell survival?
APP-null mice: are relatively normal (underweight, decreased locomotor activity, reactive gliosis)
APLP2-null mice: are relatively normal
APP/APLP2-null mice: 80% die 1 week after birth, deficits in balance and strength
Check slide 26 for diagram explanation
Production of beta-amyloid
The Aβ42 variant is more hydrophobic, more prone to fibril formation, and is the predominant isoform found in cerebral plaques.
Normally an individual would have Aβ40 at 50-70% and Aβ42 at 5-20%. However in people with Alzheimers, their Aβ42 is higher.
When the Aβ42 fragments are released from their secretory vesicles, these fragments fold to toxic form and aggregate.
Biomarkers
New criteria required as it doesn’t establish diagnostic criteria for preclinical AD. Additional research is required before this stage of AD can be diagnosed. New criteria recommend biomarker testing for MCI (Mild Cognitive Impairment).
- Total tau proteins increases to about 300%, probably as a result of
neuronal and axonal degeneration - Aβ decreases to about 50%, probably due to increased deposition in
plaques - Phosphorylated tau increases, owing to the hyperphosphorylation
associated with NFT - Combining the 3 increase sensitivity and specificity of diagnosis to
90%
What causes Alzheimers?
Cause/ causes of Alzheimers are not known.
Probs the result of many factors (rather than a single cause)
Factors include number of brain changes that begin as early as 20 years before clinical phenotype is even evident.
Brain functions normally (Initial brain changes) -> Subtle decline in congnitive function (Mild cognitive impairment) -> Obvious decline in cognitive function & memory changes (Advanced Alzheimers)
^this is according to the the 1984 criteria
Explain the continuum of Alzheimer’s Disease on a graph.
Preclinical -> MCI (Mild cognitive impairment) -> Dementia
Cognitive function decreases as the years goes on and the more the patient ages.
Check slide 30 for graph.
Describe the Hypothetical model of biomarkers graph.
Order in which biomarkers become abnormal in relation to the stage of disease progression:.
First:
(preclinical and plateaus)
1. Amyloid-β accumilation (CSF/PET)
(increasing towards the end of preclinical, then sharply & progressively increases during MCI until heightening in Dementia)
2. Synaptic dysfunction (FDG-PET/fMRI)
3. Tau-mediated neuronal injury (CSF)
4. Brain structure (volumetric MRI)
5. Cognition
(not affected in preclinical, only slightly increases towards the end of MCI but then sharply and steeply heightens and increases during the late stages of dementia)
Describe age as a risk factor.
- Biggest risk factor is age
- AD is not a normal part of aging
- Majority of cases are diagnosed after 65 years old. This is late-onset Alzheimers
- Minority of cases before 65 (early onset Alzheimers)
check slide 32 for details
Describe Apolipoprotein E (APOE) as a risk factor
- 34 kDa, 299 amino acid glycoprotein
- Gene found on chromosome 19
- Produced at high levels in liver and brain
- Synthesised by glia in brain (predominantly astrocytes)
- Present in lipoprotein particles in the CNS which are HDL-like (apoE
in the CNS comes from the CNS) - Binds to soluble and aggregated amyloid-
- Exists in 3 common isoforms in humans, e2, e3, e4
- We have 2 copies of each gene, one from each parent
- Six possible combinations e2/e2, e2/e3, e3/e3, e3/e4, e2/e4 or e4/e4
APOE4 e4 associated with higher risk. ~ ¼ population. Increases lifetime risk by up to 4
~2 % population gets a ‘double dose’ increases risk by 10
60 % population has a ‘double dose’ of e3. Approx half develop Alzheimer’s by late 80’s
APOE e2 is mildly protective. Carriers less likely to get Alzheimer’s. 11 % pop has e2/e3 and 0.5 % has e2/e2
Greatest impact on age of development.
Having gene does not guarantee a person will develop Alzheimer’s
State the other 5 risk factors.
- Family history- (individuals with a parent, brother or sister increased risk)
- Mild cognitive impairment (MCI)- (mild but measurable changes in thinking abilities)
- Cardiovascular disease- (physical inactivity, high cholesterol, diabetes, smoking, obesity)
- Social engagement/ lifestyle- (mentally and socially active, diets low in saturated fats- relatively little known)
- Head injury & traumatic brain injury- (moderate injury x2 risk, severe injury x4.5 risk)
No treatment is available to slow or stop Alzheimer’s disease
State the neuropathological hallmarks of AD
- Senile Neuritic plaques
- Neurofibrillary tangles
- Neuronal and synaptic loss
Senile plaque (amyloid) & Neurofibrillary Tangles (tau)- the 2 markers are an absolute requirement for AD.
Neuropathological basis of Alzheimer’s Disease
Alzheimer’s disease is rarely found without other neurodegenerative co-pathologies – less than half!
Macroscopic:
* Moderate cortical atrophy
* Enlarged sulci and atrophy of gyri (frontal and temporal)
* Atrophy posterior cortical areas
* Enlarged lateral ventricles
* Loss of neuromelanin pigmentation, locus coeruleus
Microscopic
* Amyloid plaques (dense core or diffuse)
– up to 8 types
* Neuritic plaques
* Cerebral amyloid angiopathy (80-95%)
* Neurofibrillary tangles (6 isoforms)
* Granulovacuuolar degeneration
* Hirano bodies (dendrites)
* Inflammatory response
* Synaptic lossNo single feature or combination of specific to
Alzheimer’s disease - can be highly suggestive
Definitive diagnosis requires microscopic examination of multiple brain regions.
Neuropathological basis of Alzheimer’s Disease
3 major subtypes of atypical AD classified by relative density of hippocampal NFT’s with respect to neocortical NFT’s.
Hippocampal Sparing AD:
* Earlier age of onset
* Rapid rate Cog decline
1 in 20 atypical
Limbic Predominant AD:
* Later onset
* APOE4
Posterior Cortical Atrophy:
* Also Hip Sparing
* Corticobasal degeneration
* Prion disease
Frontal variant AD:
* About 1 in 50
* Affects frontal lobes
Comorbidities
* Cerebrovascular pathology
- 16% AD CASES
* Lewy BodY related pathology
- Approx. 25% AD develop Parkinsonian features
* TDP-43 pathology
- found in hippocampal sclerosis and AD
-19 -75% AD
* Agyrophilic grain pathology
- 4R tauopathy
* Age related tau pathology
- primary age-related tauopathy (PART)
- age-related tau astrogliopathy (ARTAG)
* Suspected non-Alzheimer’s disease pathophysiology (SNAP)
State the common symptoms of dementia
Cognitive:
* Memory loss
* Failing intellect (inability to learn new skills)
* Poor concentration
* Language impairment
* Disorientation/confusion
Changes in personality and mood:
* Apathy
* Depression
* Delusions
* Anxiety
– Out of character behaviours
* Aggression
* Sleep disturbances
* Disinhibition
Disability:
*Difficulties with activities of daily living
*Self-neglect
*Incontinence and other physical disabilities
A. Age-related
B. Progressive
1. mild
2. moderate
3. severe
C. Incurable
Neurofibrillary tangles
Neurofibrillary Tangles:
* Tangles correlate with cognitive decline
* Tangles identified in a number of other dementias
* Tangles composed of tau protein
* Tau protein in tangles is hyperphosphorylated
Paired Helical Filaments
* Composed of paired filaments
* Twisted in a helix
* Twisted at ~ 800 Angstrom intervals
* Indefinite length
* Somatodendritic localisation
* PHFs are insoluble
Phosphorylation of tau is a key observation in the pathogenesis of Alzheimer’s disease
Tau Isoforms
- Tau gene consists of 16 exons located on Ch 17q21
- Alternative splicing gives rise to six tau isoforms
- Adult brain contains all six isoforms
- Fetal brain has a single isoform
- Relatively little secondary structure
Check slide 44 for images
Tau Schematic
Function:
* Axonal transport
* Microtubule binding
* Stability
Tau Phosphorylation
85 potential phosphorylation sites identified on tau
45 sites identified on PHFs isolated from AD brains
The order an importance of each phosphorylation site is unclear
The kinases and phosphatases responsible for each phosphorylation site remain to be
determined
No function has been assigned to any single site phosphorylation in Alzheimer’s disease
AD-related phosphorylation recapitulates normal phosphorylation during development
Phosphorylation of tau is a key observation in the pathogenesis of Alzheimer’s disease
Imbalance of tau-kinases/phosphatase activity in AD?
Check slide 47 for images
Enzymes that modify sites on tau listed above:
Glycogen synthase kinase (GSK3)
Cyclin-dependent kinase (cdk5)
p42/p44 mitogen-activated kinase (ERK1/2)
Stress-activated protein kinase (SAPK1,2a,2b,3,4)
Brain specific kinase 1/2 (BRSK1/2) Calcium and Calmodulin-dependent kinase II (CamK II)
Casein kinase 1/2 (CK1/2)
Microtubule affinity-regulating kinase (MARK)
Protein kinase N (PKN)
Dual-specificity tyrosine phosphorylation regulated kinase 1A (DYRK1A)
Mitogen and stress-activated kinase (MSK1)
p38 mitogen-activated kinase(p38 MAPK) p70S6 kinase Phosphorylase kinase
Protein kinase B (PKB/AKT) Protein kinase C (PKC)Rho kinase 90kDa Ribosomal S6 kinase (RSK1/2)
Serum and glucocorticoid-induced protein kinase 1(SGK1) Tau-tubulin kinase 1/2 (TTBK1/2) c-Jun N-terminal kinase (JNK)
Enzymes that modify sites on tau listed above
AT8 epitope S202 T205
27/10/202048 CLS6004BR1 R2 R3 R4E2 E3
Acidic region Proline-rich region Repeat region C-term
Glycogen synthase kinase (GSK3)
Cyclin-dependent kinase (cdk5)
p42/p44 mitogen-activated kinase (ERK1/2)
Stress-activated protein kinase (SAPK1,2a,2b,3,4)
Brain specific kinase 1/2 (BRSK1/2) Calcium and Calmodulin-dependent kinase II (CamK II)
Casein kinase 1/2 (CK1/2)
Microtubule affinity-regulating kinase (MARK)
Protein kinase N (PKN)
Dual-specificity tyrosine phosphorylation regulated kinase 1A (DYRK1A)
Mitogen and stress-activated kinase (MSK1)
p38 mitogen-activated kinase(p38 MAPK) p70S6 kinase Phosphorylase kinase
Protein kinase B (PKB/AKT) Protein kinase C (PKC)Rho kinase 90kDa Ribosomal S6 kinase (RSK1/2)
Serum and glucocorticoid-induced protein kinase 1(SGK1) Tau-tubulin kinase 1/2 (TTBK1/2) c-Jun N-terminal kinase (JNK)
Hyperphosphorylation of Tau in Alzheimer’s disease
Tau is hyperphosphorylated early in AD (kinases including GSK3, Cdk5, MAPK, DYRK). It impairs its ability to bind MT’s and promotes aggregration, forming NFT’s.
Check slide 49 for diagram.
Key terms
- Pretangle tau: Soluble hyperphosphorylated tau. Gallayas-negative
(Gallayas silver staining only stains highly aggregated fibrillary
material) - PHFs: Paired helical filaments, paired twisted filaments of tau
- Neuropil Threads: Short thread like structures. Mostly dendrites
which contain PHF’s. Fairly common in cases with tangles - Neurofibrillary tangles: Multiple PHFs aggregated
Staging of Alzheimer disease-associated neurofibrillary pathology
The development of intraneuronal lesions at selectively vulnerable brain sites is central to the pathological process in Alzheimer’s disease.
Lesions consist mainly of hyperphosphorylated tau protein and include pretangle material, neurofibrillary tangles, and neuropil threads.
AD-related neuropathologocal process spans decades and the distribution pattern of the lesions occur according to a predictable sequence.
Accumulation of these lesions does not appear at random.
Basic Neuroanatomy
- transentorhinal – transitional entorhinal region
- Limbic system – Collection of brain structures, include hippocampus,
entorhinal region. Variety of functions including emotion behavior
and long-term memory - Isocortex/neo-cortex – outer layer of cerebral hemispheres. Made
up of six layers labeled I – VI
Stage I (1)
The point at which tau protein begins to clump into tangles
Tangles begin to form in the transitional entorhinal cortex
Relay station between cortex and hippocampus
Stage II (2)
Tau tangles accumulate further in the trans entorhinal region (yellow)
In the brains hippocampus (pink) and neocortex (blue) tau begins to aggregate but has not formed tangles
Neuritic plaques appear in the CA1
Stage III (3)
Extensive tangles formed in transentorhinal region
Tangles begin to form in hippocampus and in neocortex tau is beginning to aggregate
A few neuritic plaques begin to form in outer layers II-IV of neocortex
Stage IV (4)
Tangles still occupy relatively small portion of brain
Lesion density increases in sites affected in Stage III
Pathology extends to mature neocortex
Dense neuritic plaques develop
Neocortex, largest part of brain and involved in higher functions
Stage V (5)
Tangles have caused extensive death
Moderate to severe dementia
Neocortex pathology extends fanlike in frontal, superolateral and occipital directions
Lesion begin to appear in previously unaffected areas
(Identify the hippocampus, transitional entorhinal region & neo-cortex using the diagram on slide 58)
Stage VI (6)
Most areas of neocortex show severe lesions
Underlying white matter contains AT8 in axons
Extensive neuronal death
Neuritic plaques widespread
Plaques and tangle distribution at different stages of Alzheimer’s
disease progression (Braak staging)
- Prodromal (Transectorhinal I- II)
- Early-Moderate (Limbic III-IV)
- Moderate-Late (Isocortical V-VI)
Check slide 60 to show the tangles and plaques develop in each stage
The Alzheimer Amyloid Precursor protein (APP) - basics
677-770 amino acids
type I trans-membrane protein
signal peptide membrane
cellular function: unknown but ubiquitously expressed
Check slide 62 for diagram including all labelling of the protein
The Alzheimer Amyloid Precursor protein (APP) - describe splicing
Alternative splicing
neuronal APP: Predominantly lacks exons 7 (protease inhibitor domain) and exon 8, always contains exon 15.
Absence of exon 15 (L-APP) activates a Chondroitin sulfate Glycosaminoglycan-attachment site.
Check slide 63 for diagram.
Amyloid precursor protein - describe processing
2 pathways- Major or Minor
Via the Major pathway, alpha cuts the APP and forms normal harmless fragments (APPs-α or C83)
Via the Minor pathway, Aβ1-42 cuts the APP and forms 3 fragments:
1. APPs-β
2. p7
3. Aβ peptide.
This Aβ peptide is what leads to Alzheimers.
APP processing – a closer look
One of the most important aspects of APP are the different steps of its degradation.
Commonly these processing steps have been grouped into the amyloidogenic (Aβ producing) and the non-amyloidogenic pathway.
Multiple proteases involved and the specifics of how they are used in this process largely defines the cellular and the pathological role of APP.
Aβ related region extracellular or cytosolic domains.
Most APP molecules are first cleaved by Aβ-secretase
…if not cleaved by α-secretase APP will be cleaved by α-secretase.
BACE – β-site APP cleaving enzyme-1 a membrane bound protease.
.. the remaining C-terminal fragments are cleaved by γ-secretase (γ= gamma)
and AD relevant Aß42 may be generated
What is γ-secretase?
Four subunits:
* presenilin (PS)1 or PS2
* nicastrin
* anterior pharynx defective
1a or 1b (APH)-1a or APH-1b
* PS enhancer (PEN)-2
All four components necessary for γ-secretase activity
Processing of APP
α secretase pathway (non-amyloidogenic):
α-secretase = ADAM 9, 10, 17
(ADAM = a disintegrin- and metalloproteinase)
β-secretase = BACE-1 (β-site APP-cleaving enzyme-1)
β secretase pathway (amlyoidogenic):
γ-secretase = a complex
presenilin (1 or 2),
nicastrin,
presenilin enhancer -2
anterior pharynx defective -1
Production of β-amyloid peptide.
In patients wit AD, there are increased Aβ42 levels, Aβ42 variant is more hydrophobic, more prone to fibril formation, and is the predominant isoform found in cerebral plaques.
Aggregation of β-amyloid peptide
Check slide 76 for diagram.
It is facilitated by metals: Zinc and Copper.
Aβ monomers, Aβ oligomers, Aβ fibrils, Diffuse plaques, Amyloid plaques
Toxic form of β-amyloid peptide - soluble or
insoluble aggregates?
Fibrillar/aggregated forms of Aβ peptide, but not soluble monomeric Aβ peptide, are toxic to cells in culture and brains of rhesus monkey.
Neurodegeneration appears prior to the appearance of plaques in mouse models of AD
Plaques are found in people without dementia or brain injury.
Explain the hereditary component of AD
Majority of AD cases are sporadic, ~200 family lines in the world that carry mutations
Hereditary forms of AD generally induce earlier onset of neurodegeneration
Most AD-related mutations cause increased formation of amyloid plaques
Pure genetic causes of AD account for < 1% of all AD cases
Mutation of Amyloid Precursor Protein (APP) in AD
14 families world wide have a genetic fault on
chromosome 21 in the APP gene:
Swedish, Flemish, Dutch, Italian, Arctic, Iowa,
French, Iranian, Austrian, Florida, London,
Indiana, Australian, Tottori
All appear to share the same characteristic of
altering APP processing such that more Aβ is
deposited
Explain the mutation of amyloid Precursor Protein (APP) in AD using the diagrams on slide 79, 80 & 81
Large number of families carry a mutation on chromosome 14
PS1 mutations account for the majority of FAD cases
> 100 mutations have been described
Very rare group of families have mutations on PS2
Familial Alzheimer’s Disease
Mutations accelerate disease progression through diverse cellular mechanisms resulting in an increase in the amyloidogenic processing of APP.
Proves amyloid is a key factor in Alzheimer’s disease, not just a bystander
Pathology of genetic and non-genetic AD indistinguishable
What is the relationship between plaques and tangles?
There a 2 models used to explain this:
1. Serial Model
2. Dual Pathway Model
What does the serial model suggest?
That increased Aβ leads to increased Tau phosphorylation. This causes synaptic loss which leads to cell loss.
What does the Dual pathway model suggest?
That both Aβ an Tau phosphorylation increase. This causes synaptic and cell loss.
This model, unlike the serial model, does not say that the cause of increase tau phosphorylation is increased Aβ.
Evidence for β-amyloid as a cause of AD
1) Increased number of plaques in brains of patients with AD
2) Genetic mutations in familial (early-onset) AD cause increased production/deposition of A peptides (e.g. APP, PS1, PS2)
3) A plaques appear in Down Syndrome patients (trisomy 21) that carry an extra copy of the APP gene (situated on chromosome 21)
4) APOE4 allele increases the risk of sporadic (late-onset) AD
5) Transgenic mice expressing mutant human APP genes develop plaques and neurodegeneration (but not extensive and no tangles)
6) Aggregated or fibrillar (but not soluble/monomeric) Aβ peptide is toxic to cells in culture and the brains of aged rhesus monkeys
Toxicity of β-amyloid
Toxicity of Aβ peptides/oligomers/fibrils/plaques on cells is not understood (different experimental conditions, model systems and endpoints)
Aβ peptides induced apoptosis in cultured cells and post-mortem brain tissue, but not in transgenic mouse models of AD
Aβ peptides induce production of reactive oxygen species, leading to cell damage and apoptosis. Cell culture, clinical and epidemiological studies suggest that the anti-oxidants Vitamin E and oestrogen are protective.
Aβ peptides may directly insert into cellular membranes and disrupt cellular integrity, act as a non-selective ion channel, or allow release of ROS
Aβ may promote the aggregation of tau to form neurofibrillary tangles
Evidence for tau as a cause of AD
- NFT’s are common to many forms of dementia and are often the only
- pathological lesion in these diseases – no conditions where there are plaques only
- NFT’s show higher correlation with AD progression than plaques
- Familial Alzheimer’s disease is due to mutations in APP processing molecules
- and result in high levels of plaques and tangles; FTDP-17 has NFT, never plaques
- Transgenic mice that express mutant human APP and develop plaques but not tangles exhibit cognitive
defects - Ab injection (or crosses with mutant APP mice) into the brains of transgenic mutant-tau mice accelerate
tangle formation - But expression of tau in fruit fly causes neurodegeneration without formation of NFT’s
- Neuronal loss in AD brain can occur in regions devoid of NFT’s
- Some FTDP-17 patients with extensive neurodegeneration have relatively few NFT’s
What is the relationship between plaques and tangles?
Leading perception: Alzheimer’s primary amyloidoses and secondary tauopathy but the tauopathy contributes to neurodegeneration ie plaque formation is initiating
event of AD progression.
Cause (e.g. gene defect) -> Aβ peptide accumulation -> Neuronal injury -> Altered Tau metabolism and/or ultimately Neuronal dysfunction and death which results in Dementia in the End
Proteolytic degradation of Aβ plaques
Two major proteases involved in Aβ (monomer) degradation are IDE and NEP
Presence of IDE or NEP decreases Aβ accumulation
Inactivating mutations of IDE and NEP have been detected in AD patients
Interestingly, while IDE is mostly cytoplasmic, NEP is a transmembrane protein whose catalytic site is located in its extracellular domain
Hereditary component of AD
Mutation of APP, PS1, PS2 (early onset) causes increased production of Aβ1-42 peptide
Mutation of ApoE (late onset) causes increased accumulation/decreased clearance of Aβ1-42 peptide
Mutation of IDE/NEP (late onset) causes decreased clearance of plaques
The amyloid cascade hypothesis: imbalance
between production and clearance
Check slide 93
Aβ Processing abnormality a common
pathway in dementia
Genetic
– APP mutations: proteins preferentially processed by amyloidogenic route
– Presenilin (γ-secretase) mutations: increased amyloidogenic processing
– Down’s syndrome
* 3, not 2 copies of APP gene
* more amyloidogenic processing
Non-genetic
– Head injury
– Ischaemia – increases amyloidogenic processing
– Unknown factors modify
neurotransmission
* affect balance between α and β/γ-secretase activity
NB pure genetic causes of AD (familial) are
<1% of all cases, typically early onset.
Therapeutic opportunities for the treatment of AD
Check slide 96 for diagram
- Production: sAPPα binds to C83, attaching its β-secretase end to the α-secretase end of C83. This causes the γ-secretase of the C99 to snap in the middle of the cell membrane allowing the C99 to fragment into AICD and Aβ
- Plaque Build up: Aβ Aggregation occurs where plaque is built up, which results in Aβ oligomerisation, which then results in either Neuronal Cell Death (which causes cognitive and behavioural abnormalities) or Clearance.
- Clearance
Amyloid based therapies
Therapeutic tactics:
Halt Aβ production
–γ-secretase inhibitors
–β-secretase inhibitors
–α-secretase stimulators?
Enhance Aβ removal
–Vaccination
–Non-vaccine plaque busters
NB pure genetic causes of AD (familial) are
<1% of all cases, typically early onset.
Reducing Aβ production
γ-secretase
*cleaves other important substrates (eg notch) hence specificity is important
Eg substrate docking sites are different so target APP binding site
Eg Likewise ATP-binding site modulates APP processing specifically
*certain nonsteroidal anti-inflammatory drugs (NSAIDs) allosterically modulate
γ-secretase to favor production of Ab40 over Ab42
*Inhibitor LY450139 lowered A in plasma but not CSF
β-secretase
lacks other substrates ( KO is viable with very minor anomalies)
and therefore is a safer target
α-secretase
stimulating its activity could decrease A since it cleaves within the peptide
Eg Bryostatin a protein kinase C inhibitor enhances activity
SufficientWhat About Translation?
Clinical Trials of Immunisation
AN1792 (Elan): A peptide designed to produce an immune response and clear plaques
* Phase I clinical trial in 2000 showed variable antibody response
* Phase IIa study saw 6% of patients develop meningoencephalitis due to T-cell mediated autoimmune responses.
– No cognitive benefits
– Trial halted
* 2008 publication of follow up study, 5 years after last inoculation (Holmes et. al. Lancet. 2008)
– Of 8 treated patients, 6 showed a reduction in plaques (biggest antibody response, greatest plaque removal)
– No apparent difference in time to progression of severe dementia
– Not well controlled data, but suggest that cognitive benefit is unlikely
* 2019 follow up study (Nicoll et. al. Brain. 2019)
– Of 14 of 16 treated patients with AD, 6 showed evidence of plaque removal
– 2 patients who had died 14 years after immunization had very sparse or no detectable plaques
– Plaque free for many years after therapeutic intervention ceased
– Tau pathological spread continued despite plaque removal
– Despite modification of amyloid pathology, most had progressed to severe dementia (due to tau propagation)
27/10/2020100 CLS6004B
Amyloid Removal Alone is Not Sufficient
Amyloid Therapies in Development
- Humanised monoclonal antibody: bapineuzumab (Wyeth – now Pfizer &
Elan)
– passive Aβ immunotherapy with bapineuzumab results in decreases in
CSF T-tau and P-tau (Blennow et al., 2012 Arch Neurol 69:1002-10)
– Phase III trial with over 1000 patients halted in summer 2012. “Drug
engages with target but fails to produce clinical benefit”. Some hope
that the outcome better in group with e4 allele.
– Is there a problem with trial design, or is it a failure of mechanism?
– Solanezumab (Eli-Lilly)
– Targets a different part of A compared to bapineuzumab.
– Reported as safe, and effective with respect to lowering CNS plaques
(Farlow et al. 2012 Alzheimers Dement. 8:261-71)
– Entering phase III trials
Amyloid Therapies in Development
- Gamma-secretase inhibitor, Semagacestat (Lilly)
- Failed phase III trials (worsened outcome)
- Off target effects limit this class of drugs, but some prospects for amyloid selectivity.
- BACE Inhibitor
- Enzyme initially considered to be “undruggable”
- Phase II/III Clinical Trial of MK-8931 (Merck Sharpe & Dohme) commenced
December 2012 - Several companies have compounds in the pipeline
