Lecture 44 - Alzheimer's Disease and Other Dementias - Pathophysiology and Pharmacology Flashcards

1
Q

Alzheimer’s disease (AD) is

A

an age-related neurodegenerative disorder
Survival following diagnosis: 3 – 20 years (mean = 8 years)
2:1 female:male (opp of PD)

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2
Q

AD is the major cause of

A

dementia

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3
Q

AD symptoms

A

(1) memory loss (especially recent memories)
(2) impaired ability to learn, reason
(3) impaired ability to carry out daily activities; confusion, untidiness
(4) anxiety, suspicion, hallucinations
(5) motor dysfunction can also occur in late-
stage disease

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4
Q

Environmental risk factors of AD

A
  • Age (major risk factor)
  • Low educational level
  • Reduced mental activity in late life
  • Reduced physical activity in late life
  • Risks for vascular disease
  • Head injury
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5
Q

AD neuropathology:

A

loss of brain volume
amyloid plaques and neurofibrillary tangles
synapse loss

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6
Q

Amloid plaques

A
  • extracellular
  • consist of amyloid-β peptide (Aβ)
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7
Q

Neurofibrillary tangles

A
  • intracellular
  • consist of hyperphosphorylated tau
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8
Q

Progression of AD neuropathology

A

largely in basal forebrain and nucleus basalis and entorhinal cortex
- AD neuropathology (plaques and tangles spreads through the cortex as the disease progresses.
- Neuropathology primarily affects areas of higher cognitive function:
→ entorhinal cortex (memory formation/consolidation)
→ hippocampus (memory formation/consolidation)
→ basal forebrain cholinergic systems (learning)
→ neocortex (memory, learning, cognition)
→ nucleus basalis (memory, attention, arousal, perception)

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9
Q

Synapse loss

A
  • A striking feature of neurons with neurofibrillary tangles and neurons in the vicinity of amyloid plaques is the destruction of synapses.
  • Synapse loss results in reduced levels of neurotransmitters – especially acetylcholine, but also serotonin, norepinephrine, and dopamine.
  • Another consequence: dysregulated glutamate –> excess excitotoxicity and neurotoxicity
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10
Q

Which is the key pathogenic molecule: Aβ or tau?

A
  • Genetic evidence suggests a key role for Aβ :
    → Mutations in the gene encoding the Aβ precursor protein, APP, are linked to early onset AD.
    → Trisomy 21 (Down’s syndrome) is associated with an AD-like phenotype in the 4th decade of life, and the APP gene is located on chromosome 21.
    → Mutations in the gene encoding presenilin proteins involved in cleaving Aβ from APP are linked to early onset AD.
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11
Q

Production of Aβ peptide from APP

A
  • Aβ peptide is released from the transmembrane amyloid precursor protein (APP) by the activity of β-secretase (BACE1) and γ-secretase.
  • Cleavage of APP by α-secretase in the middle of the Aβ segment releases a non-amyloidogenic (non-toxic) fragment.
  • Aβ is generally 40 or 42 amino acid residues in length. Aβ42 forms amyloid fibrils more readily than Aβ40.
  • Mutations in the APP gene favor cleavage by β- or γ-secretase, resulting in the production of more Aβ42 relative to Aβ40.
  • Mutations in the gene encoding presenilin-1 or presenilin-2 (PSEN1 or
    PSEN2), which are components of the γ-secretase complex, alter APP cleavage by γ-secretase, resulting in the production of more Aβ42 compared to Aβ40.
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12
Q

Effects of Aβ aggregation on tau pathology

A
  • Aβ aggregation is thought to promote tau hyperphosphorylation, leading to neurofibrillary tangle formation, cytoskeletal anomalies, and disruption of axonal trafficking.
    early aggregates can bind to cell surface receptors linked to kinases in cell –> kinase activation –> tau hyperphosphorylation –> neurofibrillary tangles –> disruption of cytoskeleton, axonal trafficking
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13
Q

Neurofibrillary tangle formation results in

A

cytoskeletal defects
In healthy areas the microtubule tracks are well organized and thus permit the trafficking of cargoes (e.g. vesicles, organelles) down the axon. This transport is critical for proper synaptic function.
In unhealthy areas where tangles have accumulated, the cytoskeletal tracks are disrupted and disorganized, resulting in defects in axonal transport that lead to synaptic dysfunction.
becomes disrupted when tau becomes hyperphosphoylated

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14
Q

Effects of Aβ aggregation on microglial activation

A
  • In addition to causing damage to neurons directly, Aβ is thought to induce neurotoxicity indirectly by triggering microglial activation, a process that is probably aimed at clearing amyloid from the brain.
  • Activated microglia release pro-inflammatory cytokines (e.g. prostaglandins, interleukins, tumor necrosis factor-α) that cause neuroinflammation.
  • Activated microglia also release reactive nitrogen species (e.g. nitric oxide, peroxynitrite) and reactive oxygen species (superoxide, hydrogen peroxide) that cause oxidative stress.
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15
Q

Impact of ApoE genetics on AD risk

A

– ApoE – responsible for transporting cholesterol in brain (LDL); altered ApoE function can affect Aβ aggregation or clearance.
– ApoE genetics:
→ There are three ApoE isoforms as a result of polymorphisms
in the ApoE gene: ApoE2, ApoE3, and ApoE4.
→ Individuals with one or two ApoE4 alleles have an increased
risk of AD, whereas inheritance of the ApoE2 (protective) allele decreases AD risk.

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16
Q

Current AD therapies: cholinesterase inhibitors

A

cleaves at ester bond, releases acetic acid and choline
- Cholinesterase inhibitors block the enzymatic reaction shown above, thereby compensating for the loss of acetylcholine that results from the degeneration of cholinergic nerve terminals in AD.
Donepezil, rivastigmine, galatamine

17
Q

Donepezil (Aricept):

A

specific, reversible inhibitor of acetylcholinesterase

18
Q

Rivastigmine:

A

inhibits acetylcholinesterase and butyrylcholinesterase (delivered orally or with a patch)

19
Q

Galantamine:

A

selective, reversible inhibitor of acetylcholinesterase and enhances the action of acetylcholine on nicotinic receptors (increases acetylcholine release from cholinergic neurons)

20
Q

Normal cholinergic synapse vs Alzheimer’s disease synapse

A

normal cholinergic synapse: balance between free released ACh and AChe enzymes
cholinergic synapses - alzheimer’s disease: less ACh being released, but same # of active ACh enzymes

21
Q

Current AD therapies: anti-glutamatergic therapy

A

memantine

22
Q

Memantine

A

NMDA antagonist that blocks glutamatergic neurotransmission via a noncompetitive mechanism, reduces excitotoxicity.
- Glutamate is an excitatory neurotransmitter that is required for learning and memory.
- Excess glutamate signaling leads to excitotoxicity, a mechanism that can result in neuronal death.
- Combination drug, memantine ER + donepezil: Namzaric
memantine does not impair neurotransmission or plastic processes

23
Q

Strategies for disease-modifying therapy to delay disease emergence

A

Research is focused on discovering agents that target the following:
(i) Aβ generation (β- and γ-secretase inhibitors)
(ii) Aβ aggregation (e.g. inositol, polyphenols [resveratrol], peptides)
(iii) Aβ clearance (vaccines, Aβ antibodies – aducanumab, lecanemab, donanemab: Aβ aggregate-specific antibodies). - work in early stages and clear away plaques
(iv) tau kinase inhibitors (lithium, valproate) - to prevent tau hyperphosphorylation
(v) glutamate-mediated excitotoxicity (troriluzole,
which induces expression of glial glutamate transporter GLT-1). - to clear glutamate from synapse
(vi) inflammation or oxidative stress (anti-inflammatory agents [NSAIDS], dietary antioxidants [vitamin E, polyphenols])

24
Q

Imaging – Florbetapir (18F)

A

monitors AB and tau pathology in brain
* Radiolabeled agent that binds β-amyloid, visualized by PET scanning
* May help to clarify role of amyloid in pathophysiology and lead to improved treatments.
* Radiolabeled agent specific for tau: 18F-Flortaucipir

25
Q

Biomarker analysis – AD blood test

A
  • Previous methods for detecting Aβ deposition in the brain: Aβ PET and measurement of Aβ in the CSF.
  • New blood tests involve immunoprecipitation coupled with mass spectrometry or antibody-based detection and are designed to measure plasma levels of Aβ isoforms.
  • Aβ levels detected in plasma using these tests correlate well with Aβ PET burden.
26
Q

-Non-AD dementias

A

vascular dementias
dementia with lewy bodies
frontotemporal dementia (Picks’ disease)

27
Q

Vascular dementia

A
  • Impaired judgment or executive function is a more common initial symptom than the memory loss characteristic of AD.
  • Occurs as a result of brain injury associated with vascular disease or stroke.
28
Q

Dementia with lewy bodies

A
  • Combination of cognitive decline and parkinsonian symptoms
  • Visual hallucinations are a core diagnostic feature.
  • Neuropathology is characterized by the presence of cortical Lewy bodies.
29
Q

Frontotemporal dementia (Picks’ disease)

A
  • Disinhibited behavior
  • Neuropathology is characterized by the presence of tau accumulations (Pick’s bodies in the case of Pick’s disease)