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

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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