Lecture 3: Prions Flashcards
What are prion diseases?
A family of diseases that cause neurodegenerative disease characterised by prolonged incubation periods.
What are prion diseases associated with?
Transmissible spongiform encephalopathies
What are symptoms of prion diseases?
Ataxia (loss of coordination) due to effects on cerebellum
Rapidly progressing dementia and death within a few week or a few years
True or false: prion diseases can only be sporadic (no underlying cause)?
False, they can also be acquired (contact with infectious agent) or inherited (mutations in the prion protein)
Give four human prion diseases
- Kuru
- Creutzfeldt-Jakob disease (CJD)
- Gerstmann-Straussler-Scheinker syndrome (GSSS)
- Fatal familial insomnia (FFI)
What is Scrapie?
Prion disease that affects sheep and goats
- formally healthy animals would begin to lose weight and rub against rough surfaces
- motor disturbances
- tremor
- animals could become very fearful or aggressive
What is BSE?
Bovine Spongiform encephalopathy/mad cow disease
- thought to be caused by cattle feed being made using scrapie infected sheep meat and prion proteins resistant to normal processing procedures (heat or acid) used to inactivate proteins
What was the cause of the Kuru epidemic among the Fore people in Papua New Guinea in the 1950’s?
ritual cannibalism - eating prion infected material - acquired prion disease
True or false: there is evidence of person to person transmission in the acquisition of CJD?
False
However, there is evidence of iatrogenic transmission (due to surgical procedure) - certain transplants, grafts, transfusions from cadaveric donors have go one to develop CJD, or cerebral electrodes used in brain surgery
What is new variant CJD (vCJD or nvCJD)?
BSE transmitted to humans from consumption of infected cow meat
What are the symptoms of CJD?
Loss of balance and coordination
Slurred speech
vision problems and blindness
abnormal jerking movements
progressive loss of mobility
loss of cognition and memory
change in personality (fearful or aggression)
Why do most people with CJD die within a year of symptoms apprearing?
usually due to infections caused by immobility
- muscle weakness lead to further issues such as increased risk of developing pneumonia
What are the neuropathological changes that occur in prion diseses?
Vacuolation (prev referred to as spongiform changes) in grey matter and spinal cord - holes where neurons are missing
PrP-positive amyloid plaques and scrapie-associated fibrils (SAF)
Gliosis (proliferation of astrocytes and activation of microglia - inflammation - immune response in response to neuronal death and damage, as well as plaque formation)
Florid plaques - plaques surrounded by vacuolation so see death of neurons and synapses
What is an amyloid plaque?
aggregation of an abnormal protein
(prion protein in prion disease, beta-amyloid protein in AD)
What made researchers realise that the infectious agent of scrapie was unusual and not a ‘conventional virus’?
agent was found to be resistant to UV and ionising radiation
resistant to nuclease digestion and chemical procedures that modified nucleic acid
resistant to heat and autoclaving
resistant to formaldehyde (was used to sterilise surgical instruments)
How did they discover the scrapie infectious agent was a prion protein?
Brain homogenates from scrapie and non-infected animals were run on a western blot. When the brain homogenates ere treated with proteinase K, the band at 27-30 kDa in the scrapie infected animals remained (resistant to digestion by proteinase K)
- further experiments showed that this band has a rod like structure that looked like the amyloid fibres but no nucleic acid
what is PrP^C and describe its structure and properties?
normal cell-surface glycoprotein,
non-infectious,
secondary structure dominated by alpha helices,
Easily soluble and can be digested by proteases
What is the physiological role of PrP^C?
unknown - highly expressed in neurons and may be involved in copper transport, superoxide dismutase-like enzyme function and maintenance of long-term memory
What is PrP^Sc and describe its structure and properties?
Scrapie form of the prion protein
infectious
Secondary structure dominated by beta-sheets
Insoluble and highly resistant to digestion proteases
Form aggregates
True or false: PrP^C and PrP^Sc have the same amino acid sequence?
True
What effect does the PrP^Sc have on PrP^C?
PrP^Sc can convert PrP^C to PrP^Sc
What is the main difference between the normal prion protein and the scrapie form?
Secondary structures (glycosylation) leading to alpha helical conformation of the normal prion protein and beta sheet conformation of the scrapie form
What evidence confirms the central role of PrP in prion disease?
- the infectious agent remains resistant to nuclease digestion
- infectious fractions contain PrP^Sc and no other scrapie-specific macromolecule
- PrP^C knock-out mice are resistant to scrapie
- amyloid in the brain is formed from PrP aggregates
- mutations and polymorphisms in the PrP gene cause familial CJD, GDD and FFI in humans, control incubation period/susceptibility, cause prion disease in transgenic mice
- infectious prions have been reconstituted from recombinant PrP
how are prions believed to replicate?
Scrapie prion present by spontaneous generation, inherited mutation or inoculation/infection with scrapie agent.
This scrapie prion comes into contact with normal endogenous prion and causes the conversion of these monomers to the scrapie form which start to accumulate
The prion scrapie version with high beta-sheet structure facilitates aggregation. monomeric forms come together and act as seed to draws in other monomeric forms resulting in amplification and expansion into fibril like structure.
Breakdown of larger fibrils form new seeds (nucleation site) that attract more PrP^C monomers into the scrapie like form that then enter the replication cycle.
what size fragment is generated when PrP^C is digested with proteinase K?
27-30 KDa fragment
What factor may make the PrP^C protein more susceptible to transformation to the scrapie version?
mutations within the PRNP gene
What is the name of the gene that encodes the normal PrP^C prion protein and what chromosome is it located on?
PRNP gene located on chromosome 20
What is the significance of the M129V polymorphism within the PRNP gene?
this polymorphism is a key determinant of susceptibility and age of onset for both sporadic and acquired prion disease
- whether an individual expresses methionine or valine is important in determining if they will get the prion disease and if so how young they will be
What is one of the key polymorphisms that is associated with prion disease?
M129V polymorphism within the PRNP gene
What are the six major classification of sporadic CJD based on?
The expression of methionine or valine at codon 129 and the molecular weight of the PrP^Sc after proteinase K digestion
What is the difference between type 1 and type 2 sporadic CJD (sCJD)?
Type 1 = 21KDa scrapie prion after proteinase K digestion
Type 2 = 19KDa scrapie prion after proteinase K digestion
What are the six major classification of sporadic CJD?
MM1, MM2, VV1, VV2, MV1, MV2
What are the most common subtypes of sporadic CJD?
MM1/MV1
True or false: the different subtypes of sCJD always have the same clinical presentation and pathology?
False: they can have distinct clinical presentation and pathology
What is the only acquired prion disease known in humans?
vCJD (only form of human prion disease known to be transmitted from animal to human upon consumption of contaminated beef after being fed scrapie-contaminated sheep products as well as blood transfusions)
How does ago of onset differ between vCJD and sCJD?
the age of onset for vCJD is much younger (mostly under 50) than that of sCJD (mostly over 55)Wh
What are causes of sporadic prion disease?
- somatic mutation in PrP^C open reading frame
- RNA editing error
- protein translational error
- spontaneous conversion of PrP^C to PrP^Sc (proteins undergo different steps through maturation into tertiary structure that could go wrong and convert the prion protein)
- horizontal transmission from other infected individuals of the same or different species.
What is a prion strain?
infectious isolates that when transmitted to identical hosts, exhibit prion-disease phenotypes
What differs between prion strain?
different clinical and pathological signs of prion disease
the different prion strains, when digested with proteinase K will show distinct patterns of digest on western blot
different incubation times, histological lesion profiles and specific neuronal target areas
What is adaptation that prions exhibit?
The amount of time that it takes between inoculating the animal and observing the symptoms onset decreases with each passage (incubation time decreases between each animal inoculated/with every infection)
What is species transmission barriers of prions?
Take prion from mouse and infect a sheep = doesn’t infect sheep
take sheep homogenate injected with the mouse prion and inject back into another mouse, the mouse will develop the prion protein (suggests the prion is replicating within the sheep or maintaining its scrapie like phenotype but not actually infecting the sheep)
What evidence supports the association between BSE and vCJD?
- people with vCJD are younger than those with sCJD
- there is a temporal sequence between outbreaks of BSE and developing of vCJD (incubation period)
What are the treatments of prion diseases?
symptomatic - treat motor and cognitive issues as best you can but there are currently no effective therapies for prion disease
What are possible disease-modifying treatment targets for prion diseases?
- decrease production of PrP^C
(downside is that we dont know what the cellular PrP protein is doing so may affect cell in other way) - prevent conversion of cellular prion into scrapie version using antibodies
- prevent aggregation of scrapie prion using antibodies or compounds that prevent aggregation, or compounds that prevent breakdown of fibrils to prevent seeding of new aggregates)
- try and clear degraded fibril
- try and block the toxicity (challenging bc we don’t know why the scrapie prion is neurotoxic)
What are some possible intracellular targets of prion disease treatment?
- enhancers of autophagy (bringing the aggregates back into the cell for lysosomal degradation)
- enhancement of the UPS
- chaperone therapy
What have yeast prions told us about how prions may affect gene expression?
PrP^Sc-like form in S.cerevisiae can aggregate into amyloid fibres and prion particles can be passed onto daughter cells during cell division.
The scrapie-like prions can cause stop codon readthrough (not respond to the stop codon resulting in increased translation that would not have occurred under normal conditions) = protein-based epigenetic elements - epigenetic modulation of gene expression.
