Transcript Protein Misfolding Can Have Deadly Consequences 992328張謹淳

Protein Misfolding Can
Have Deadly
Consequences
992328張謹淳
992334林思慧
992504曹紋寧
992539鄭翰群
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• In April 1996 a paper was published in the
medical journal Lancet that generated
widespread alarm in the populations of
Europe.
• The paper described a study of 10 persons
afflicted with Creutzfeldt-Jakob disease (CJD),
a rare, fatal disorder that attacks the brain,
causing a loss of motor coordination and
dementia.
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CJD
• CJD can occur as an inherited disease.
• CJD can also be acquired.
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• The cases described in the 1996 Lancet paper
had also been acquire, but the apparent
source of the disease was contaminated beef.
• The contaminated beef was derived from
cattle raised that had contracted a
neurodegenerative disease that caused the
animals to lose motor coordination and
develop demented behavior.
• Mad cow disease!
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• A disease that runs in families can invariably
be traced to a faulty gene, whereas diseases
that are acquired from a contaminated source
can invariably be traced to an infectious agent.
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• How can the same disease
be both inherited and
infectious?
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In 1960, D.Carleton Gajdusek
showed that the Islanders were
contracting a fatal
neurodegenerative disease.
They called it”kuru”.
Autopsies of the brain of patient
who had died of kuru showed a
distinct pathology , referred to as
spongiform encephalopathy.
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• It was soon shown that the brains of islanders
suffering from kuru were strikingly similar in
microscopic appearance to the brains of
persons afflicted with CJD.
• This observation raised an important question:
Did the brain of a person suffering from CJD,
which was known to be an inherited disease,
contain an infectious agent?
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• In 1968, Gajdusek showed that when extracts
prepared from a biopsy of the brain of a
person who had died from CJD were injected
into a suitable lab animal, that animal did
indeed develop a spongiform encephalopathy
similar to that of kuru or CJD
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• Clearly, the extracts contained an infectious
agent, which at the time was presumed to be
a virus.
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• In 1982, Stanley Prusiner published
a paper suggesting that, unlike
viruses, the infectious agent
responsible for CJD lacked nucleic
acid and instead was composed
solely of protein. He called the
protein a prion.
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• It was presumed initially that the prion
protein was an external agent, some type of
virus-like particle lacking nucleic acid.
• Contrary to this expectation, the prion protein
was soon shown to be encoded by a gene
(called PRNP) within the cell’s own
chromosomes.
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• The gene is expressed within normal brain
tissue and encodes a protein designated PrPC
(standing for prion protein cellular) that
resides at the surface of nerve cells.
• A modified version of the protein(PrPSc) is
present in the brains of humans with CJD.
• Unlike the normal PrPC , the modified version
of the protein accumulates within nerve cells,
forming aggregates that kill the cells.
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• In the purified states, PrPC and PrPSc have very
differentphysical properties.
PrPC
PrPSc
Monomeric molecule
fibrils
Soluble in salt solutions
Insoluble in salt solutions
Readily destroyed by
protein-digesting enzymes
Resistant to enzymatic
digestion
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PrPC
PrPSc
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How PrPSc act an infectious agent?
• PrPSc bind to normal protein(PrPC) can convert
PrPC into PrPSc
• Appearance of the abnormal protein in the
body starts a chain reaction
→Normal protein molecules gradually
converted to the abnormal prion form.
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PrPC
PrPSc
converted
PrPSc
Bind again
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Alzheimer’s disease (AD)
• Memory loss
• 10% individuals at least 65years of age
40% are 80 years or older
• The brain contains fibrillar deposits of an
insoluble material referred to as amyloid.
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Alzheimer’s disease (AD)
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© 2000 - 2012 American Health Assistance Foundation
http://www.ahaf.org/alzheimers/about/understanding/plaques-and-tangles.html
CJD v.s AD-same
• Fetal neurodegenerative disease
• Occur in an inherited or sporadic form
• Fibrillar deposits result from self-association
of a polypeptide
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CJD v.s AD-differences
• Proteins that form the disease-causing
aggregates are unrelated.
• Affected parts of the brain are different.
• AD is nontransmissible.
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AD-amyloid hypothesis
• Amyloid β–peptide (Aβ):part of amyloid
precursor protein(APP)
• Aβ is cut by Β-secretase and γ-secretase from
APP
• Two species of Aβ: Aβ40 & Aβ42
• Misfold Aβ42 has the potential to cause
damage to the brain, the soluble oligomers
are toxic to nerve cells
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AD-Aβ42
• Refold into a different conformation (contains
β–pleated sheet)
• Tend to self-associate to large aggregates
(visible fibrils)
• Oligomers appear to attack the synapses and
lead to the dead of nerve cell
• Inherited AD leads to increased Aβ42
production, by mutations in the APP gene
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drugs
• Management of symptoms
• Three strategies :
– Prevent formation of Aβ42
– Remove the Aβ42
– Prevent interaction between Aβ
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• One of the best approached to the
development of treatments for human
diseases is to find laboratory animal,
particularly mice, that develop similar
diseases.
• Animals that exhibit a disease that mimics a
human disease are termed animal model
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• For whatever reason, the brain of aging mice
show no evidence of the amyloid deposits
found in humans, and there was no animal
model for AD.
• Until 1995, the researchers create a strain of
mice by genetically engineering the mice to
carry a mutant human APP gene.
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• This strain develop amyloid plaques in their
brain and performed poorly at tasks that
required memory.
• We can use these investigations to illustrate
some of the steps required in the
development of a new drug.
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• In 1999, Dale Schenk and his colleagues
published an extraordinary finding.
• Repeatedly injecting the animals with the very
same substance that causes the problem, the
aggregated Aβ42 peptide.
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• The researcher had immunized (i.e.,
vaccinated) the mice against the disease.
• When young (6-week-old) mice were
immunized with Aβ42, they failed to develop
the amyloid brain deposits as they grow older.
• When older (13-mounth-ole) mice whose
brains already contained extensive amyloid
deposits were immunized with the Aβ42.
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• The immunized mice performed better than
their nonimmunized littermates on memorybase tests.
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Phase I clinical trial
• A Phase I clinical trial is the first step in testing
a new drug or procedure in humans.
• Phase I tests are designed to monitor the
safety of the procedure rather than its
effectiveness against the disease.
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• No one showed any ill-effects due to injection
of the amyloid peptide. As the result, the
investigators were allowed to proceed to a
Phase II clinical trial.
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Phase II clinical trial
• Phase II:
1. the patients are randomly divided into two
groups that are treated similarly except that one
group is given the curative factor being
investigated and the other group is given a
placebo.
2. The study is double-blinded, which means that
neither the researchers nor patients know who is
receiving treatment and who is receiving the
plcebo.
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• The Phase II trial for the Aβ vaccine began in
2001 and enrolled more than 350 individuals in
the United States and Europe who had been
diagnosed with mild to moderate AD.
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• Most of these patients were successfully
treated with steroids, but the trial was
discontinued.
• 2001 was reported in 2008,analysis of this
patient group indicates that Aβ42 vaccination
had had no effect on preventing disease
progression
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Two interpretation
• Amyloid deposits are not the cause of the
symptoms of dementia.
• Irreversible toxic effect the deposits had
already occurred by the time immunization
had begun.
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• These treatments had stared earlier,the
symptoms of the disease might never have
appeared.
• It may be possible to begin preventive
treatments in persons who are at very haigh
risk of developing AD before they develop
symptoms.
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Drugs develop
• Alzhemed:small molecular designed to bind to
Aβ peptide and block certain
interactions,thereby stopping molecular
aggregation and fibril formation
• Flurizan:a nonsteroidal anti-innammatory
drug,or NSAID,like ibuprofen.
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• The drug was found to reduce mental decline
over a period of one year by an average of 81
percent compared to patient receiving a
placebo.
• The drug is now being test in larger Phase III
study
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