23. Prions Flashcards
7 types of prion disease
- Kuru
- Creutzfeld-Jacob Disease (CJD)
- Variant Creutzfeld-Jacob Disease (VCJD)
- Gerstmann-Straussler-Scheinker Syndrome (GSS)
- Fatal Familial Insomnia (FFI)
- Mad-Cow Disease
- Bovine Spongiform Encephalopathy
what does “kuru” mean?
Shivering
6 steps of kuru disease progression
- Uncontrollable twitching
- Decreased muscular coordination/cerebellar ataxia
- Decreased ability to walk
- Dementia
- Muscle atrophy and paralysis
- Death
what is Kuru also called?
laughing illness
what does Kuru brain look like? (3 hallmarks)
- Lots of vacuolar spongiform degeneration of neurons
- Kuru plaques
- No inflammation
what type of material builds up in Kuru brain?
fibrous material
symptoms and pathology of kuru are IDENTICAL to:
Creuzfeldt-Jakob disease (CJD)
symptoms and pathology of kuru are SIMILAR to:
- Gerstmann-Straussler-Scheinker Syndrome (GSS)
- Fatal Familial Insomnia (FFI)
is kuru familial?
yes, families have high incidence of spongiform encephalopathy
what was the hypothesis for Kuru being caused by a gene? why?
sex-linked gene with age variation of incidence and gender
because mostly children and women were affected
is Kuru caused by a gene? why?
NO! Too high penetrance in population, a small community with such a gene would not survive
what happened when Kuru brain tissue was inoculated in various animals?
animals were NEGATIVE when reacted serum from affected patients against the tissue samples
what is the disease in sheep called?
Scrapie
5 symptoms of scrapie
- trembling
- ataxia
- weakness
- strange behaviour
- paralysis, death
histopathology of scrapie (3)
- spongiform encephalopathy without inflammation
- kuru plaques
- microfibrillar arrays
how was spongiform encephalopathy found to be transmissible?
2-5 years after the initial negative results were found, the animals who were initially inoculated were found to have spongiform encephalopathy
describe the only way to measure/detect kuru or scrapie infection initially
inoculate material into chimpanzee brains , then observe for 2-5 years
describe the new way to measure/detect kuru or scrapie infection
inoculate material into HAMSTER brains, then observe for 30-45 days –> much faster!!
what was found from inoculated brains?
analyzed tissue fractions on SDS-PAGE gels –> found a protein band that was not found in uninfected brains
what was weird about the infectious material inoculated in brains?
no viral agent or nucleic acid was found in infectious material
how did they know that the infectious material did not contain nucleic acid?
treatments that inactivated nucleic acids did not affect infectivity
what 4 treatments can be used to inactivate nucleic acids?
- UV light
- psoralens
- X-irradiation
- DNAse
what type of material was the infectious material?
protein
how did they know that the infectious material was protein?
treatments that inactivated proteins abolished the infectivity
2 examples of treatments that inactivate proteins
- chemical denaturants (alkali, phenol, SDS)
- proteases
3 components of the prion hypothesis
- infectious agent made entirely of protein
- no nucleic acid
- PRoteinacious agent of infectION
what is the infectious agent called?
Scrapie Prion Protein (PrP)
how did they find the gene for PrP? 5 steps
- Purified the protein band from SDS-PAGE
- N-terminal aa sequencing
- reverse translated the aa sequence into a nucleotide sequence
- used nucleotide sequence to make a probe
- used probe to find PrP gene
what encodes for PrP protein?
host cell mammalian gene –> PrP gene
what is the role of PrP gene?
function is unknown but we know it is constitutively expressed as a membrane protein
what are 4 examples of tissues that PrP is expressed in
- neurons
- lymphocytes
- follicular DC
- intestinal epithelium
what happens in PrP KO mice?
normal phenotype
describe PrP in uninfected hosts
PrP migrates as a 33-35 kd protein (PrPc)
describe PrP in infected hosts
an additional protein migrates as a 29-32 kd protein (PrPsc)
what happens when PrPsc is treated with Proteinase K?
leaves a proteinase-resistant core PrP fragment
what happens when PrPc is treated with Proteinase K?
PrPc is completely digested
in general, how does PrPsc differ from PrPc?
post-translational modifications
5 post-translational modifications in PrPsc
- glycosylation at 2 extra sites
- removal of N-terminal signal peptide sequence
- removal of C-terminal hydrophobic segment
- addition of C-terminal phosphatidylinositol glycolipid
- removal of various non-signal N-terminal aa residues
2 consequences of PrPsc post-translational modifications
- 3D folding
- physical and biological properties
is there a covalent difference between PrPc and PrPsc?
no
difference in 3D structure of PrPc vs PrPsc
PrPc = 40% alpha helix, barely any beta sheet
PrPsc = 20% alpha helix, 50% beta sheet
what is the consequence of the different 3D structures of PrPc and PrPsc?
PrPc is soluble
PrPsc is insoluble –> forms plaques in brain
what is Prusiner’s statement of prion hypothesis?
A misfolded form of a protein can catalyze the refolding of other PrP molecules into the same misfolded conformation
describe the nucleation-dependent polymerization model of PrPsc formation
- normally, conversion between PrPc and PrPsc is reversible but PrPsc is less stable than PrPc
- PrPsc aggregates promote conversion of PrPc by binding to and stabilizing the unfavoured PrPsc
- therefore, requires an initial nucleation process by a seed that initiates aggregation
what does the nucleation-dependent polymerization model not explain?
nucleation-dependent polymerization model does not explain familial disease
what is the second model for PrPsc formation?
- PrPsc would be more stable than PrPc but kinetically inaccessible
- PrPsc would catalyze the rearrangement of PrPc to more stable PrPsc
- infection would depend on ability of PrPsc to bind and catalyze conversion of PrPc
what model does the second model of PrPsc resemble?
resembles the model of proteins where proteins are separated from true free energy minima by a large barrier
in the second model, what would spontaneous cases of CJD result from?
CJD would result from a slow rate of spontaneous conversion of PrPc to PrPsc
in the second model, what would familial syndromes result from? 2
mutations that:
1. increase populations of unstable intermediates
2. increase rate of spontaneous conversion of PrPc to PrPsc
what was causing the spread of Kuru?
RITUAL CANNIBALISM
women and children ate brains –> therefore more spread of PrPsc
what happened to Kuru once cannibalism stopped?
once cannibalism stopped, Kuru also stopped
what caused the spread of bovine spongiform encephalopathy and mad cow disease?
in early 1980s, process for dealing with diseased animals changed so scrapie-infected material entered the animal feed food chains –> scrapie was found in bovines
what happened to cattle affected by mad cow disease? (8 symptoms)
- nervousness
- aggression
- abnormal posture
- lack of coordination
- difficulty rising
- decreased milk production
- loss of weight despite appetite
- death
which species did BSE cross into? what is this disease called?
crossed into humans to cause Creutzfeld-Jakob Disease
distinct strains of prions can produce: (4)
- distinct and reproducible patterns of incubation time
- distinct patterns of CNS involvement
- distinct glycosylation and protein cleavage patterns of PrPsc
- distinct conformational misfolded structures of PrPsc
is variant CJD the same as sporadic CJD?
no
3 differences between vCJD and CJD
- vCJD has distinct pathology
- vCJD has younger age of onset
- vCJD has psychiatric symptoms
difference btwn vCJD and scrapie
vCJD caused by diff prion strain that was less restricted by species barrier than scrapie
