Beta Amyloid Peptide: November 2009

Amyloid Cascade in Alzheimer’s Disease Review Article Shankar P S

Address for correspondence:
P S Shankar,
Emeritus Professor and Director,
MR Medical College, Gulbarga, Karnataka.

Introduction
Alzheimer’s disease (AD), the most common
cause of dementia, is a progressive and fatal
neurodegenerative disorder characterized pathologically
by atrophy of the cerebral cortex and hippocampus,
with intraneuronal neurofibrillary tangles containing
abnormally phosphorylated tau protein, extracellular
amyloid plaques, and neuronal cell death, and clinically
by gradual impairment of memory.
1 The patient gradually
becomes progressively impaired in both cognitive and
functional capacities. The loss of intellectual abilities
is of sufficient severity to interfere with social and
occupational functioning.
Memory destroying illness
The sensory experiences received by the human
brain are processed and stored as memory. This
information is recalled in an integrated fashion at an
appropriate time. Memory fades in Alzheimer’s disease
and often it is compared to the erasure of a computer
hard disk. Initially it involves failure to recall the recent
events though the person is able to recollect the events
that had taken place long ago. As the illness progresses,
the old memory also gets disappeared and ultimately
the patient fails to recognize the near and dear. This
memory destroying illness is associated with loss of
a lifetime memories that make up the identity of the
person.
Pathological process
AD is associated with destruction of more than
100 billion neurons and their associated 100 trillion
connections. There is progressive loss of cortical
neurons and formation of amyloid plaques, intraneuronal
neurofibrillary tangles and accumulation of a beta-
amyloid in arterial walls of cerebral blood vessels
(amyloid angiopathy). Beta-amyloid is the major
component of the plaques, whereas hyperphosphorylated
tau protein is the major constituent of the neurofibrillary
tangles. The pathological process of atrophy begins in
the hippocampus and spreads to involve diffuse areas
of temporal, parietal and frontal lobes of the cerebral
cortex. There is symmetric enlargement of the third
and fourth ventricles. The loss of neurons, especially
in the nucleus basilis causes a relative deficiency of
acetylcholine to result in different clinical manifestations.
Cholinesterase inhibitors
The neurotransmitter acetylcholine is necessary
for clear thinking. It gets destroyed by the enzyme
acetylcholinesterase (ChE). Since acetylcholine
deficiency has been observed in AD, ChE inhibitors
have been used to block the action of ChE so as to
increase the cerebral concentration of acetylcholine
essential for synaptic transmission. Donepezil,
rivastigmine and galantamine (ChE inhibitors) facilitate
an increase in the level of acetylcholine.
2
These agents
show improvement in global function and reduce
cognitive disturbances. There is reduction in behavioural
disturbances and temporary stabilization of activities
of daily living.
3 As the destruction of the neurons
proceeds relentlessly, the medications become ineffective
after some time.
Mementine
The symptoms of AD are thought to be due to
persistent activation of central nervous system N-
methyl-D-aspartate (NMDA) receptors by the amino
acid glutamate. Glutamate acts as the main excitatory
neurotransmitter substance. Memantine, an NMDA
receptor antagonist acts either by interfering with
glutamate excitotoxicity or by providing symptomatic
improvement through effects on functions of hippocampal
neurons.
4 Though it slows the cognitive decline in mild-
to moderate AD, its effects also do not last long.
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Journal of The Indian Academy of Geriatrics, Vol. 4, No. 1, March, 2008
New approaches to therapy
Efforts are being made to find treatment to slow
or halt the memory destroying disease following better
understanding of the molecular events that appear to
trigger this disorder. It has kindled the hope of effectively
slowing or stopping the gradual loss of neurons in the
brain, and ultimately to stop the progression of the
disease. Many drugs are under various stages of
clinical trials and there are some promising preliminary
results.
5
Amyloid plaques and tangles
A cascade of events pertaining to amyloid, underlie
development of AD.
6 Amyloid cascade hypothesis is
based on the fact that plaques and tangles of proteins
in the cerebral cortex and limbic system deleteriously
affect the higher functions of the brain. The plaques
are deposited outside the neurons and are composed
of a small protein called amyloid beta (A-beta). The
tangles are found inside neurons, and their branching
axons and dendrites. They are made up of filaments
of proteins called tau. The plaques and tangles are
responsible for the degeneration of the neurons.
Amyloid-beta triggers the disruption and death of the
neurons.
This hypothesis has led to the efforts of developing
drugs to inhibit the production of A-beta and tau, and
thus stop the harmful effects of these on the neurons.
A-beta is a short peptide. It is derived from the
amyloid precursor protein (APP) with a part of the
protein lying inside the cells and a part outside, sticking
out of the cellular membrane. Two protease enzymes-
beta secretase and gamma secretase are able to carve
out A-beta from APP. This is a normal process occurring
in all cells in the body.
In AD, there is an excess accumulation of A-beta.
Initially beta secretase cuts APP found outside the
cellular membrane with the help of aspartic acids. Then
the presenilin protein, a component of the gamma-
secretase enzyme cuts the remaining portion of APP
found inside the membrane and releases A-beta into
the aqueous environment outside the membrane, and
gets attached to one another as small soluble assemblies
(plaques). They are toxic to the neurons. Experimentally
it has been shown that high concentrations of A-beta
molecules in a test tube can assemble into fibrillary
structures similar to those found in the plaques of AD.
They have been shown to be toxic to neurons cultured
in petridishes.
The step wise process of oxidation and lipid
peroxidation of cell membranes, glutamatergic
excitotoxicity, beta-amyloid aggregation, inflammation
and tau hyperphosphorylation, cause neurotoxicity,
neuronal cell death and neurotransmitter deficit.
Neuritic plaques have a central core of insoluble
deposit of amyloid beta-peptide surrounded by
astrocytes, microglia and dystrophic neuritis consisting
of paired helical filaments.
7 Neurofibrillary tangles are
made up of paired helical filaments of abnormally filled
and phosphorylated tau protein in the neuron and its
dendrites. More tau tangles are seen in the brain as
the disease advances. There is also reduction in
synaptic density, loss of neurons and degeneration in
hippocampal neurons. There is a specific degeneration
of neurons concerned with maintenance of specific
transmitter cysteines and result in deficits of
acetylcholine, nor-epinephrine, and serotonin.
8 Though
plaque formation arrests, plaque formation of tangles
continues. It correlates with the progress and severity
of dementia.
Genetic predisposition: Members of the families
having a high risk of getting AD at a relatively young
age, carry rare genetic mutations that encode APP
specifically affecting the areas of the protein in and
around the A-beta region. Genetic predisposition appears
to be inherited as an autosomal dominant trait with
relatively complete penetrance. Four different genes
have been identified to be involved in the heritable form
of the disease. The presence of the apolipoprotein E4
allele found on the long arm of chromosome 19
increases the likelihood of development of AD.
9
Apolipoprotein E4 genotype appears to enhance A-beta
peptide aggregation or decrease its cleavage. This
makes them susceptible to develop the disease at a
relatively young age. It has been shown persons with
Down’s syndrome (trisomy 21) exhibit much higher
incidence of AD in middle age. This is due to the fact
chromosome 21 contains APP gene. There is an
increased production of A-beta from birth, and
consequently an increased amyloid deposit beginning
from a young age.
Mutations in two related genes called presenilin
1 and 2 lead to occurrence of severe form of AD very
early in life The mutations increase amount of A-beta
that is prone to clumping mutations of presenilin-1 gene
located on chromosome 14 which may lead to an early29
Journal of The Indian Academy of Geriatrics, Vol. 4, No. 1, March, 2008
onset autosomal dominant AD.
10 Rarely the mutations
of the presenilin-2 gene on chromosome 1 may cause
autosomal dominant AD with an earlier onset of the
disease and a shorter, more rapidly progressive course.
The proteins encoded by the presenilin genes are part
of the gamma secretase enzyme that help in the
synthesis of the harmful peptides.
Protease inhibitors: It is not clear how A-beta
destroys the neurons. Aggregates of A-beta found
outside the neuron can initiate a cascade of events
that can bring about an alteration of the tau protein
inside the cell. A-beta aggregates are likely to bring
about changes in the kinases that add phosphates onto
proteins. There is likelihood of addition of an excess
amount of phosphates to tau, resulting in formation of
twisted filaments. The altered tau proteins are likely
to act deleteriously by disrupting the microtubules
carrying proteins along the axons and dendrites, and
kill neurons. Thus A-beta plays the pivotal role in the
initiation of AD. In this background, drugs are being
produced targeting the proteases (protease inhibitors)
that produce A-beta, and to inhibit their activity.
The proteases use aspartic acids to catalyze
protein cutting reactions. Small-sized beta-secretase
inhibitors are yet to be developed that can effectively
pass through the blood brain barrier. Gamma secretase
is the other enzyme involved in the formation of A-beta
by cutting the remaining portion of APP inside the cell
following the cleavage by beta secretase. Studies in
mice have shown deletion of presenilin-1 gene genetically
decreases the cutting of APP by gamma secretase.
It has been proved that the protein encoded by the
gene is essential for the function of the enzyme.
Inhibitors of aspartyl proteases could block gamma-
secretase cleavage of APP in cells. Gamma secretase
also contains a pair of aspartic acids as in beta-
secretase and are essential for catalyzing the protein
cutting reaction.
11 Presenilin protein acts like an
unusual aspartyl protease in the cell membranes. The
inhibitors of gamma secretase are relatively small
molecules that can penetrate blood brain barrier.
Inhibitors of aspartyl proteases could block gamma-
secretase cleavage of APP in cells. Gamma secretases
like beta secretases contain a pair of aspartic acids
essential for catalyzing the protein cutting reaction.
Presenilin protein appears to be an unusual aspartyl
protease attached to the cell membrane. Two aspartic
acids in presenilin lie within the membrane. They are
very essential to the gamma secretase cleavage to
produce A-beta. Inhibitors of gamma secretase bind
directly to presenlin. Gamma secretase enzyme plays
an important role in maintenance of undifferentiated
precursor cells in different parts of the body. Gamma
secretase cuts a cell surface protein called Notch
receptor. High doses of gamma secretase inhibitors
cause toxic effects in mice by disrupting the Notch
signal. Molecules have been identified that modulate
gamma secretases so that A-beta production is
blocked without affecting cleavage of Notch.
11 Attempts
have been made to produce inhibitors that can curtail
the creation of A-beta or create a shorter peptide that
does not clump easily. Such a preparation called,
Flurizan has shown promising results.
Immunization: The second strategy is to clear
the brain of toxic assemblies of A-beta after its
production by active immunization. It involves recruiting
the patients own immune system to attack A-beta.
Injection of A-beta into mice genetically engineered to
develop amyloid plaques stimulated an immune response
that prevented the plaques from forming in the brain
of young mice and cleared plaques already present in
older mice.
11
The mice produced antibodies that recognize A-
beta and the antibodies made the microglia in brain
to attack aggregates of the peptide. It improved learning
and memory. However studies in humans, has lead to
development of encephalitis probably through the action
of T cells. However, immunization produced antibodies
against A-beta and there was some improvement in
memory and concentration. Passive immunization by
injecting the antibodies into patients aims to clear the
peptides. These antibodies produced in mouse cells
and genetically engineered to prevent rejection in
humans are unlikely to evoke occurrence of encephalitis
as they do not trigger T cell response in the brain.
The procedure was able to remove A-beta from the
brain.
Immunization with selected parts of A-beta instead
of entire peptide can stimulate the antibody producing
B cell of the immune system without triggering T cells
involved in the occurrence of encephalitis.
Non-immunological strategy: Non-immunological
strategy to stop aggregation of A-beta compounds has
been attempted. The compounds interact directly with
A-beta to keep the peptide dissolved in the fluid outside
brain neurons preventing formation of harmful clumps.
Alzhemed, a small molecule apparently mimicking30
Journal of The Indian Academy of Geriatrics, Vol. 4, No. 1, March, 2008
heparin binds to A-beta and reduces peptide aggregation
and shows some improvement in cognitive functions
of patients with mild AD.
Targetting Tau: The tau filaments cause neuronal
tangles, and they are a promising target to prevent
degeneration of neurons. Inhibitors could block the
kinases that place an excessive amount of phosphates
onto tau, which is an essential step in filament
formation. No success has been seen in production
of such a drug. It is hoped such drugs might work
synergistically with those targeting A-beta.
Reduction in production of APP: Cholesterol
lowering agents (statins) used to cut risk of heart
disease could become a treatment for AD. Epidemiologic
studies have shown people taking statins have a lower
risk of acquiring AD By lowering cholesterol these
drugs may reduce production of APP or perhaps affect
the creation of A-beta by inhibiting activity of the
responsible secretases. Attempts are made to prevent
AD by using statins.
Cell-based therapy: Cell therapy is another
approach in the treatment. The gene encoding a large
protein such as nerve growth factor (NGF) was inserted
into the skin biopsies obtained from patients with mild
forms of AD. Such genetically modified cells were
implanted surgically into the forebrain of the patients,
with a view that the implanted cells would produce and
secrete NGF, thus preventing the loss of acetylcholine
producing neurons and improve memory. The treatment
was associated with slowing down of cognitive decline.
Medical fraternity is eagerly looking forward for a
break through in the research to provide a drug that
could effectively slow or stop the gradual loss of
neurons. The treatment targeting A-beta may halt the
occurrence or retard progress of Alzheimer’s disease.
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