Alzheimer Disease

Learning Objectives The student should be able

1.To be familiar with the Clinical description of AD

2. Acquire knowledge of Genetics involve.

3. Pathophysiology

4. Biomarkers and animal models

5. Therapeutic approaches (Match Hypothesis)

The year 2006 is the centenary of the famous presentation of Alois Alzheimer which first described the neuropathology of Alzheimer’s disease (AD).

Alzheimer described the results of his postmortem studies on a 51-year-old-female patient known as Auguste D., who had suffered from a progressive presenile dementia

Described a relatively young patient who had developed a rapid loss of memory and had become disoriented in time and space.

Alois Alzheimer (b. 1864–d. 1915)

Alois Alzheimer

As the illness progressed, she became bedridden and incontinent. She died four and a half years after the onset of illness.

Post-mortem examination revealed an evenly atrophic brain with striking neurofibrillary pathology.

Alzheimer also described the presence of unusual deposits in the cortex that were refractory to staining.

His famous paper (Alzheimer 1907)

The Anatomical Hallmark of Alzheimer’s Pathology:Amyloid Plaques and Neurofibrillary Tangles in Brain

Amyloid Plaquescontain large amounts of a 42 amino acid peptide termed “-amyloid”, or A42

In the next lecture, we note that -amyloid itself is the initial cause of the pathophysiology that leads to dementia. Amyloid plaques probably contribute to the later stages of pathology

Neurofibrillary tangles: rich in cytoskeletal proteins, especially the microtubule-associated protein, “tau”. In the tangles: heavily phosphorylated proteins, which may cause aggregation and precipitation of the cytoskeleton.

Also generally reduced brain volume, especially in entorhinal cortex and hippocampus

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Alzheimer’s Disease

Alzheimer’s disease is the most common form of dementia. accounting for 50–60% of all cases.

Dementia is a syndrome that exhibits impaired short-term and long term memory as its most prominent feature.

Forgetfulness is the primary complaints of patients.

1. Symptoms of Alzheimer’s Disease

1. AD begins with a “pure” impairment of cognitive function. “mild cognitive impairment” does not always lead to dementia.

2. ProgressionA. AD begins slowly. At first, the only symptom may be mild forgetfulness.

In this stage, people may have trouble remembering recent events, activities, or the names of familiar people or things. They may not be able to solve simple math problems. They may begin to repeat themselves every few minutes in conversation.

B. In the middle stages of AD, individuals may forget how to do simple tasks, like brushing their teeth or combing their hair. They can no longer think clearly. They begin to have problems speaking, understanding, reading, or writing.

C. Late stage: AD patients may become anxious or aggressive, or wander away from home. Eventually, patients need total care.

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Incidence of Alzheimer’s disease (AD)

AD is the most common degenerative brain disease (est. 5 million USA, 25 million globally)

Risk Factors: age

(However, AD is not considered a normal part of aging). The more common form of AD, known as late-onset or sporadic AD, occurs later in life,

with no obvious inheritance pattern. However, several risk factor genes may interact with each other to cause the disease.

Most common risk factor gene identified so far for late-onset AD, is a gene that makes one form of apolipoprotein E (apoE). ApoE4 (prevalence ~ 16%) is the risk factor gene, 3-4 fold dominant increase.

Familial AD, which is rarer, usually starts at age 30 - 60.

65-74 75 80 >85

~5% ~10% ~20% ~50%

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Red circles: presenilin 1 and APP mutations associated with familial AD

Hardy and Selkoe, 2002, Science

presenilin 1 is part of -secretase, a membrane-associated protease

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All known genetic risk factors predisposing to Alzheimer’s disease Increase accumulation of Aβ peptides

Chromosome Gene defect Phenotype

21 β-APP mutations ↑All Aβ peptides, or Aβ40 peptidesA673T↓ Aβ peptides, AD, cognitive decline

19 ApoE4 polymorphism(ε4 allele)

↑Density of Aβ plaques & vascular deposits

14 Presenilin 1 mutation ↑Production of Aβ42 peptides

1 Presenilin 2 mutation ↑Production of Aβ42 peptides

6 TREM2 ↑Density of Aβ plaques

“The precise meaning of the amyloid hypothesis changed over the years, and differs among scientists. Originally, it was thought that the actual amyloid is pathogenic—hence the term “amyloid hypothesis”. The more current version of this hypothesis posits that Aβ (especially Aβ42) microaggregates—also termed “soluble Aβ oligomers” or “Aβ-derived diffusible ligands” (ADDLs)—constitute the neurotoxic species that causes AD” – Sheng, 2012

Abnormal states of tau mediate some effects of β-amyloid. This stage may be distal to the more toxic dimers and oligomers.

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There are “tauopathies” as well,

Mutations in tau protein.

These cause frontotemporal dementia with parkinsonism,

linked to Chromosome 17

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Animal models:

mice overexpressing APP, especially with AD-associated mutations.

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4. Biomarkers for AD.

Only an autopsy is conclusive, but progress in two areas:

1. Cerebrospinal fluid analyses of tau, phospho-tau at position 181, and Aβ42.Individual values, or ratios among these.

2. A positron emission tomography (PET) marker, [18F]Florbetapir, binds to plaques containing β-APP.

“A negative scan indicates sparse to no neuritic plaques and is inconsistent with a neuropathological diagnosis of AD at the time of image acquisition; a negative scan result reduces the likelihood that a patient’s cognitive impairment is due to AD. A positive scan indicates moderate to frequent amyloid neuritic plaques; neuropathological examination has shown this amount of amyloid neuritic plaque is present in patients with AD, but may also be present in patients with other types of neurologic conditions as well as older people with normal cognition. [Florbetapir] is an adjunct to other diagnostic evaluations.”

Negative Positive

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Cholinergic hypothesis

The cholinergic hypothesis in Alzheimer’s disease states that degeneration of cholinergic neurons in the basal forebrain nuclei causes disturbances in presynaptic cholinergic terminals in the hippocampus and neocortex, which is important for memory disturbances and other cognitive symptoms.

Potentation of the activity of the central cholinergic pathway is one strategy for the symptomatic treatment of cognitive dysfunction in AD.

Acetylcholinesterase inhibitorsTacrine

Tacrine, the first agent approved for symptomatic treatment of mild to moderate AD

Inhibit both acetylcholineesterase and butyrylcholineesterase (BuCHE)

Low bioavailability, short half-life (multiple doses)

Tacrine

Side effect: Nausea, vomiting , diarrhea,

abdominal pain.

Elevation of alanine aminotransferase (ALT) levels

Donepezil

Second generation cholinesterase inhibitors

Selective Acetylcholinestearse than (BuCHE)

Completely bioavailability, once daily

Side effect: cholinergic activity (nausea, diarrhea, headache)

Rivastigmine

Inhibit activity of both AChE and BuChE by binding to esteratic site of both enzymes and slowly dissociates.

Called Pseudo-irreversible

Twice daily

Galantamine

Inhibit AChE. allosteric modulation of nicotinic

acetylcholine receptors

It stimulates nicotinic receptors at a site distinct from that stimulated by acetylcholine action that does not rely on the presence of Ach

Metabolized by CYP2D6

Glutamate theory

Glutamate is the major excitatory neurotransmitter in the central nervous system.

An over activation of glutamate receptors, and particularly of N-methyl-D-aspartate (NMDA) receptors, leads to an immediate rise in calcium ions (cell death)

Memantine blocks glutamate-mediated excitotoxicity

Glutamate theory

Memantine was approved for the treatment of moderate and severe AD case as early as in February 2002.

The basis for this approval was the result of two randomized placebo-controlled clinical studies that have showed a positive effect in a later stage of this disease (Reisberg et al. 2003).

It only affects pathophysiological conditions (NMDA receptor over activation) and leaves physiological neurotransmission unchanged.

The amyloid cascade hypothesis

Amyloid precursor protein (APP) is a protein containing 770 amino acids

Cleaved into peptides by three enzymes: alpha, beta and gamma secretase.

APP is mainly formed by a two step process:

If alpha secretase initially cleaves APP, alpha-soluble APP (α-sAPP) is formed and eventually becomes a benign peptide.

When beta secretase initially cleaves APP, it becomes beta soluble APP (β-sAPP). β -sAPP can subsequently be cleaved by gamma secretase at

two different sites producing harmful peptides such as Ab 40 and Ab 42.

The amyloid cascade hypothesis

There are two types of beta secretase beta-site APP cleaving enzyme 1,2 (BACE 1, BACE 2)

Two forms of gamma secretase (presenilin 1, presenilin 2).

Inherited Genetic Factors

Mutations in genes involved in amyloid beta processing (seen in familial early onset cases)

Variation in a gene (apolipoprotein) that produces a protein essential for clearing cholesterol and other molecules out of the bloodstream — seen in the general population (sporadic AD cases)

Less than 10% of the AD cases are autosomal dominantly inherited and are linked to one of three different chromosomes.

To date, mutations in the following genes have been described to be causative for early onset AD

presenilin-1 gene on chromosome 14, presenilin-2 gene on chromosome 1 amyloid precursor protein gene on

chromosome 21.

Alzheimer and genetics

People that carry mutations in any one of these genes usually experience a very early onset of AD, well below 60 years of age

Potential target in the future

β-secretase inhibitors

γ-Secretase inhibitors

Metal Ions and Amyloid β proteins

Aβ itself can act as a metalloprotein displaying high affinity for copper (Cu2+) and zinc (Zn2+).

Interaction between amyloid and metal ions might mediate amyloid aggregation and amyloid nerve cell toxicity

Ion chelators

clioquinol is a hydrophobic compound that acts as a copper and zinc chelator

It can readily cross the blood brain barrier

Clioquinol was used decades ago as an oral antiamoebic compound, but it has been withdrawn from the market because of possible neuropathic side-effects.

Phase II clinical trial

Disease-related changes of the tau

tau is a protein involves binding and stabilization of microtubule structure and function.

The microtubule network in the cell is required for the transport of proteins.

Tau, calpains and apoptosis

cyclin-dependent kinase 5 (cdk5), which promote phosphorylation of tau

p35 is a neuron-specific activator of cdk5

conversion of p35 into p25 by calpain-dependent proteolysis causes prolonged activation and mislocalization of cdk5.

Consequently, the p25 ⁄ cdk5 kinase hyperphosphorylates tau, disrupts the cytoskeleton, and promotes apoptosis of primary neurons.

Tau, calpains and apoptosis

preaggregated Aβ induced the generation of a neurotoxic 17-kDa tau fragment

prevented by a calpain inhibitor

Prevented by anti-tau.

Apolipoprotein E4 (ApoE4)

Approximately 15% of the human population inherit an allele, apolipoprotein E4 (ApoE4) which can increase the risk for AD by approximately 3-fold .

The APO E gene comes in three flavors, the epsilon 2, epsilon 3 and epsilon 4 alleles.

Apolipoprotein E4 (ApoE4)

In healthy people, the frequency of the epsilon 4 allele is 10%; in AD patients, this frequency is increased to over 40%.

The existence of one or two copies of the epsilon 4 allele increases of the AD onset in a dose-dependant manner

Apolipoprotein E4 (ApoE4)

This allele increases cholesterol concentrations and may be responsible for augmenting the amount of Aβ or decreasing its clearance .

the formation of myelin is dependent on cholesterol, it has been suggested that cholesterol may be partially responsible for the progression of AD

Statins for Alzheimer

Medications which inhibit 3-hydroxy-3-methylglutaryl- coenzyme A (HMG-CoA) reductase have been proven to reduce serum cholesterol, and low density lipoproteins (LDL)

lovastatin, simvastatin and cerivastatin cross BBB reduce the amount of Aβ peptides by reducing cholesterol from the blood and/or the cerebrospinal fluid (CSF)

Anti-inflammatory

Inflammation is also occurring during the development of AD (Rogers et al. 1992; Akiyama et al. 2000).

senile plaques attracting activated microglia, reactive astrocytes, cytokines and complement components (Akiyama et al. 2000)

Reduction in the risk of AD associated with a chronic use of non-steroidal anti-inflammatory drugs (NSAID) ibuprofen, indomethacin ,but no other NSAID decrease the release of Aβ

Treatment of behavioural signs

Behavioural signs, such as aggression, psychomotor agitation, and psychosis (hallucinations and delusions)

Atypical antipsychotic drugs

Dementia and Alzheimer's disease

Alzheimer's disease (AD) is a common age-related dementia, distinct from vascular dementia associated with brain infarction.

The main pathological features of AD comprise amyloid plaques, neurofibrillary tangles and a loss of neurons (particularly cholinergic neurons of the basal forebrain).

Amyloid plaques consist of the Aβ fragment of amyloid precursor protein (APP), a normal neuronal membrane protein, produced by the action of β- and γ-secretases. AD is associated with excessive Aβ formation, resulting in neurotoxicity.

Familial AD (rare) results from mutations in the genes for APP, or presenilin genes (involve in γ-secretases function) , both of which cause increased Aβ formation.

Mutation in lipoprotein ApoE4 increase the risk of developing AD probably by interfering with Aβ clearance

Neurofibrillary tangles comprise aggregates of a highly phosphorylated form of a normal neuronal protein (Tau). Hyperphosphorylated Tau and Aβ act synergistically to cause neurodegeneration.

Loss of cholinergic neurons is believed to account for much of the learning and memory deficit in AD.

Anticholinesterases (tacrine, donepezil, rivastigmine) give proven, though limited, benefit in AD.

Clinical use of drugs in dementia Acetylcholinesterase inhibitors and NMDA antagonists detectably

improve cognitive impairment in clinical trials but have significant adverse effects and are of limited use clinically. They have not been shown to retard neurodegeneration.

Efficacy is monitored periodically in individual patients, and administration continued only if the drugs are believed to be working and their effect in slowing functional and behavioural deterioration is judged to outweigh adverse effects.

Acetylcholinesterase inhibitors Donepezil, galantamine, rivastigmine. Tacrine is also effective, but may cause liver damage. Unwanted cholinergic effects may be troublesome.

Used in mild to moderate Alzheimer's disease. NMDA receptor antagonists For example, memantine Used in moderate to severe Alzheimer's disease.

Certain anti-inflammatory drugs, and also clioquinol (a metal chelating agent), may retard neurodegeneration and are undergoing clinical evaluation.