Download Biochemistry of neurodegenerative diseases and prions

Biochemistry of
neurodegenerative diseases
and prions
Alice Skoumalová
Neurodegenerative disease
Patologic protein (amyloid)
Alzheimer‘s disease
Amyloid β
Parkinson disease
α-synuclein
Frontotemporal dementia
Tau
Hungtington disease
Huntingtin
Creutzfeldt-Jakob disease
Prion
α-helix
β-sheet
Conformational change
the starting point is the natural protein
folded in the native and active conformation
normal protein is rich in α-helix
conformations (folded structure)
the end-point is the same protein adopting
prevalent β-sheets structure
it is disease-associated protein (misfolded
structure)
Aggregation
Gain of toxic activity
Neurodegenerative
diseases
Loss of biological
function
DNA
Ubiquitin
RNA
Ribosome
ATP
Chaperones
Native protein
Chaperones
Misfolded protein
Aggregate/fibrillar amyloid
Proteasome
Accumulation
(Amyloidoses)
Degraded protein
Gain of toxicity
(Alzheimer disease)
Loss of protein function (Cystic fibrosis)
Amyloid fibril structure
Straight, unbranched, diameters in the range of 8-16nm
Composed of two to six protofilaments
Rich in a type of β-sheet structure (the β-sheets are perpendicular to the
fibril axis and bind together by the hydrogen bonds)
Molecular factors in amyloid formation
Protein misfolding is central to amyloid formation
Protein stability- the resistance of the folded conformation to misfolding- is an
important factor in determining susceptibility to amyloid formation
Destabilizing factors:
1. Extreme environments in the body, such as acidic cell compartments
2. Proteolytic removal of a portion of a protein by an endogenous protease
3. Mutations that alter the primary structure (many of the amyloid diseases
involve amino acid substitutions in an amyloid precursor protein)
4. Interactions with lipid bilayers (Aβ)
Protein quality control in the cell
Histopathological hallmarks of Alzheimer disease
Neurofibrillary tangles
Amyloid (senile) plaques
 hyperphosphorylated protein tau
protein β-amyloid (Aβ) fibrils
the abnormal processing of APP
(β-amyloid precursor protein)
APP processing and Aβ production
Amyloid cascade hypothesis of Alzheimer disease
Missense mutations in APP, PS1, or PS2 genes
Increased Aβ42 production and accumulation
Aβ42 oligomerization and deposition as diffuse plaques
Subtle effects of Aβ oligomers on synapses
Microglial and astrocytic activation (complement factors, cytokines)
Progressive synaptic and neuritic injury
Altered neuronal ionic homeostasis; oxidative injury
Altered kinase/phosphatase activities
tangles
Widespread neuronal dysfunction and cell death with transmitter deficits
Dementia
Prions
Prions are proteinaceous transmissible pathogens responsible for a
series of fatal neurodegenerative diseases (in humans,
Creutzfeld-Jakob disease and kuru, in animals, bovine
spongioform encephalopathy)
A prion (proteinaceous infectious particle, analogy for virion) is a
type of infectious agent that does not carry the genetic
information in nucleid acid!
Prions are proteins with the pathological conformation that are
believed to infect and propagate the conformational changes
of the native proteins into the abnormally srtructured form
Disease name
Natural host
PrP isoform
Scrapie
Sheep, goat
OvPrPSc
Transmissible mink
encephalopathy (TME)
Mink
MkPrPSc
Chronic wasting disease
(CWD)
Elk, mule deer
MDePrPSc
Bovine spongioform
encephalopathy (BSE)
Cattle
BovPrPSc
Feline spongioform
encephalopathy (FSE)
Cat
FePrPSc
Exotic unguale encephalopathy
(EUE)
Greater kudu, nyala
NyaPrPSc
Kuru
Human
HuPrPSc
Creutzfeldt-Jakob disease
(CJD)
Human
HuPrPSc
Gerstmann-StrausslerScheinker syndrome (GSS)
Human
HuPrPSc
Fatal familial insomnia (FFI)
Human
HuPrPSc
PrPC
PrPSc
The normal protein
The abnormal, disease-producing protein
is called PrPC (for cellular)
is called PrPSc (for scrapie)
is a transmembrane glycoprotein
(neurons, lymphocytes); its function is
unknown; it binds Cu2+ (regulation its
homeostasis)
has the same amino acid sequence
(primary structure)
has dominant secundary structure βsheets
has dominant secundary structure αhelix
is insoluble
is easily soluble
is multimeric and resistant to digestion by
proteases
is monomeric and easily digested by
proteases
is encoded by a gene designated PRNP
located on the chromosome 20
When PrPSc comes in contact with PrPC,
it converts the PrPC into more of itself
These molecules bind to each other
forming aggregates
Molecular models of the structure of:
PrPC
Predominantly α-helix (3)
PrPSc
β-sheets (40%), α-helix (30%)
Prion aggregates (an electron microscope picture)
Replication cycle
The presence of an initial PrPSc: exogenous (infectious forms)
or endogenous (inherited or sporadic forms)
This first prion will initiate PrPSc accumulation by sequentially
converting PrPC molecules into PrPSc in replication cycle
PrPSc molecules aggregate
Mechanism of prion progression
Prion diseases: rare neurodegenerative disorders (one person per million)
1. Sporadic (85 %)
In the sixth or seventh decade, rapidly progressive (death in less than a year)
Creutzfeldt-Jakob disease (CJD)
2. Familial (inherited-15%)
Mutations in the PrP gene that favour the transition from the cellular form to the
pathological form of PrP
Gerstmann-Straussler-Scheinker disease (GSS), fatal familial insomnia (FFI)
3. Transmissible (rare; a source of great concern)
In the laboratory, disease can be readily transmitted to mice by intracranial
injection of brain homogenate taken from prion-infected animals
Propagation of kuru disease in New Guinea natives (ritualistic cannibalism)
Recently, it has been discovered that BSE had been transmitted to humans in
Europe after consumption of infected beef, producing a variant of the CJD
called vCJD
Transmissible spongioform encephalopathy (TSE)=prion disease
is a group of progressive conditions that affect the brain and nervous
system of humans and animals and are transmitted by prions
The pathology: vacuolar degeneration, neuronal loss, astrocytosis and
amyloid plaque formation
The clinical signs: loss of motoric functions (lack of coordination, ataxia,
involuntary jerking movements), personality changes, depression,
insomnia, confusion, memory problems, dementia, progressive tonic
paralysis, death
Definitive diagnostic test: biopsy of brain tissue
There is no cure
Prion transmission
1. Direct contact with infected tissues
CJD has been transmitted:
-to patients taking injections of growth hormone harvested from human
pituitary glands
-from instruments used for brain surgery (prions can survive the autoclave
sterilization process)
-in corneal grafts
-in electrode implants
2. Consumption of affected tissues
Kuru was transmitted through cannibalism in Papua New Guinea
Humans can contract the disease by consuming material from animals
infected with the BSE (vCJD)
How can prions make their way through the gut and into the brain?
Proteins normally are digested down to amino acids in the gut
Hypothesis: They circumvent the normal process of intestinal absorption by
passing into the the Gut-Associated Lymphoid Tissue (GALT)
Prion strains
=subclassification of prions following the structural
characteristics that define their pathological profile
The various prion diseases differ in incubation times and
neuropathologic profiles, although in all cases the same
misfolded protein (PrPSc) is responsible
The existence of various prion strains: although they are
chemically identical, they differ in their exact conformations
The structural characteristics of individual prion strains are
propagated via the replication cycle
These characteristics appear to direct the precise tissue
targeting, incubation time, and pathogenesis of the strain
Consumption of BSE-infected meat
BSE prion
Variant CJD: younger cohort,
distinct clinical
characteristics
vCJD prion is identical to
BSE prion
Classical CJD: older cohort
CJD prion is different
Strains can be differentiated by
characteristic incubation periods and
neuropathology
-distinct biochemical characteristics
Properties of a single strain may be
retained after passage in a range of
different species
-when re-isolated in the original host
Conformational selection and transmission
barriers
A wide range of PrPSc conformations
A subset is compatible with each individual PrP
Transmission between species with same conformations
Therapeutic strategies
1. Compounds can be designed to specifically disrupt the replication cycle of
the PrPSc
Design of such compounds had proven successful in cell-based models but
must now be extended to animal models and human clinical trials
2. Vaccine design: The abnormally folded proteins expose a side chain of
amino acids which the properly folded protein does not expose.
Antibodies specifically coded to this side chain amino acid sequence
stimulate an immune response to the abnormal prions
3. Design of peptides that break the β-sheet structures
4. Gene therapy: modification of the prion gene
Genetic engineering research: cattle lacking a necessary gene for
prion production - thus theoretically making them immune to BSE
(December 2006)
Characteristic
Classic CJD
Variant CJD
Median age at death
68 years
28 years
Median duration of
illness
4-5 months
13-14 months
Clinical signs and
symptoms
Dementia; early
neurologic signs
Prominent
psychiatric/behavioral
symptoms; delayed
neurologic signs;
sensoric hallucinations
Specific changes on MRI Often present
Often present
Specific changes on
EEG
Often absent
Immunohistochemical
analysis of brain tissue
Often present
Variable accumulation of
the PrPSc
Marked accumulation of
the PrPSc
Presence of agent in
lymphoid tissue
Not readily detected
Readily detected
Presence of amyloid
plaques in brain tissue
Often present
Often present
Summary
Conformational defect of proteins - protein misfolding diseases neurodegenerative diseases
Oligomeric aggregates - neurotoxic
The mechanism: oxidative stress, disruption ion homeostasis
The prions are proteins that carry information for self-reproduction (contradict
the central dogma of modern biology)
The prions are expressed in cells of healthy humans and animals; their
abnormal conformations (PrPSc) are insoluble, resistent to digestion
and aggregate
The PrPSc attacks the native prion PrPC, changes its conformation into an
abnormal form and causes an exponential production of insoluble
proteins; they aggregate and form the fibrillar structure
Prion disease are rare fatal degenerative disorders; a portion of them can be
transmitted; this mechanism is not clear (e.g. transmision of BSE to
human)