Prion disease Flashcards
Define neurodegenerative disease
Group of related disorders with a variety of pathologies and aetiologies that are all characterised by progressive loss of function and eventual neuronal death.
Associated with ageing.
2 common features of neurodegenerative diseases?
Progressive synapse loss
Build up of misfolded protein aggregates in the brain.
What are prion diseases?
Also known as Transmissible spongiform encephalopathies (TSEs): rarer forms of neurodegeneration.
Features: accumulation of misfolded protein and progressive synapse loss as well as - spongiosis (holes in tissue) and astrocytosis (overproliferation of astrocytes).
Evidence of glial change in prion disease?
GFAP: marker of astrocytes, showed increased number of astroctyes in infected.
Iba1: marker of microglia, showed increased number of microglia in infected.
How does EEG of infected differ?
In late stages of disease: characteristic periodic sharp wave complexes (PSWC).
Aguglia et al., (1977): case studies of two women with CJD who developed PSWC but later lost PSWC shortly before death. Hypothesised that this could be due to increased GABA mediated inhibition that reduces amplitudes of evoked potentials before death in CJD.
Why can’t we say that the spread of neurodegenerative diseases through protein misfolding is specific to prion disease?
Other neurodegenerative have prion-like propagation of misfolded protein.
Prion: PrP
AD/dementia: amyloidBeta and tau
Other tauopathies: tau
Parkinson’s: alpha-synuclein
What is the worsening of neurodegenerative diseases often correlated to?
Increased protein accumulation
Describe Scrapie
First described prion disease: found in sheep.
Infected: loss of coordination, uncontrollable urge to itch, ataxia, progressive paralysis and certain death.
How was scrapie proved to be transmissible?
Cuillé and Chelle, 1936: performed intraocular inoculation (basically used brain homogenate) of brain tissue from an affected animal into two healthy sheep who subsequently developed.
Further demonstrated by accidental inoculation: vaccine targeting looping ill virus was created from lymphoid tissue of scrapie-infected animals and went on to infect hundreds of sheep that received vaccine.
Describe kuru
Human prion disease that was devastating the Fore people of Papua New Guinea that pathologically resembles scrapie.
Confirmed to be infectious by transferring the disease to chimpanzees.
Transferred via ritualistic cannibalism: tribe ate dead to show love for them.
Eradicated after abolishment of practice.
Describe BSE
Bovine Spongiform Encephalopathy: unsure whether crossed species barrier from sheep or occurred sporadically but the number of cases greatly increased when infected carcases were processed into cattle feed.
Infected cattle fodder was also fed to other animals and resulted in new TSEs (cats, greater kudu, cheetahs ostriches and more!)
Naturally, led to concerns of transmission to humans: new form of human prion disease CJD (Creutzfeldt-Jakob disease) was discovered and termed variant CJD (vCJD).
Describe Creutzfeldt-Jacob disease
Most common human prion disease and is also known as subacute spongiform encephalopathy or neurocognitive disorder due to prion disease.
Always terminal: rapidly progressing dementia, development of movement disorder such as tremor, most have characteristic PSWC.
Death after 4 months.
What are the different types of CJD disease?
1) sporadic: 90%, unknown stim results in formation of PrP Sc.
2) Genetic: 10%, genetic abnormality in Prnp gene.
3) Transmitted: rare
4) Iatrogenic: v rare - specific to surgery as very hard to clean medical. If had brain surgery after infected there is risk.
NB: overall there are 3 forms of prion disease with transmitted and iatrogenic combined into acquired category.
Outline vCJD
Clinically and pathologically resembles CJD but has:
Longer duration (with dementia occurring much later)
Prolonged psychiatric symptoms
Only affected young people (mean onset 29 but patients as young as 16 developed it)
Frequently EEG changes were absent
What is the molecular mechanism of prion disease?
Prnp gene encodes PrP protein of which there are 2 isoforms, same primary structure but altered secondary and tertiary.
1) PrP C: normal endogenous, secondary mainly alpha helices
2) PrP Sc: mutated, secondary mainly beta pleated sheets = insoluble and resistant to protease degradation.
PrP Sc is infective as initiates the conversion of PrP C into more PrP Sc. As brain becomes depleted of c it stimulates synthesis of more and means more substrate for pathological conversion.
Why is cross species infectivity uncommon?
Ability of PrP Sc to recruit and convert PrP C is reduced or abolished due to variances in amino acid sequence.
PrP function?
Prnp encodes: Ubiquitously expressed gylcosylphosphatidylinositol (GPI) anchored glycoprotein that resides at the cell surface.
Role unknown but tightly developmentally regulated and highly conserved between mammalian species - suggests essential biological function.
Prnp KO?
However, Prnp knockout mouse models have been generated and shown to be developmentally and phenotypically normal (Bueler et al.,1992; Manson et al.,1994a). This could be due to compensatory mechanisms.
Subtle alterations: impaired LTP and reduced afterhyperpolarisation potentials. Weakened GABA A mediated fast inhibition.
So overall could be role in modulation of neuronal excitability.
Even adult onset conditional knockout mice remain healthy. In these mice the Cre-LoxP system was used to excise the Prnp gene 9-10 weeks after birth (Mallucci et al.,2002).
KO mice cannot get prion disease which supports idea that PrP Sc is infectious agent
Primary rat neurons cultured with PrP C?
Increased axon, dendrite and synapse formation and suggest role for PrP in maintenance of hippocampal networks
How do you destroy prions?
It is very difficult to destroy prions and this poses risks not only for transmission of human prion as previously mentioned but also for animal cases i.e. BSE and how to dispose of carcasses etc. Known as zombie proteins as if not completely denatured they fold back into correct confirmation and re-nature.
Disposing in landfill and burning: not good option as prions leached into soil and groundwater.
Time: sheep reintroduced into a unihabited and supposedly decontaminated sheephouse after 16 yrs. They contracted scrapie which seems to be infectious after 16 yrs.
Protease degradation, radiation, fixation with formaldehyde: not possible due to size and structure of prion proteins.
NaOH: at high conc works as hydrolyses peptide bonds and destroys tertiary structure.
Outline prion animal model
Rocky Mountain Laboratory (RML): mouse prion is inoculated into mouse brains. A true model of prion disease unlike other models (AD) where human mutant proteins expressed as prions naturally found in mouse species.
Characteristic spreading of prion that initiates in the hippocampus: leads to memory and learning deficits.
*) They do not burrow, this is due to motivational deficits.
*) Infected mice explore novel objects to same degree as familiar as cannot remember old object.
Astrocytosis is also present in the hippocampus.
Outline prion animal model timescale of progression.
At 0 weeks: prion injection.
16 weeks: synapse loss.
18: protein synthesis decline.
20: neuronal loss (seen by spongiosis in brain)
24: terminally ill.
Why are tg37 transgenic mouse models often used?
tg37 overexpresses PrP at approx 3 times wildtype levels.
They display the same characteristic features of synapse loss, neuronal loss, and behavioural deficits, in a stereotyped accelerated timescale. They typically become terminally sick by 12 weeks post inoculation.
Why are repurposed drugs studied for prion disease?
Developing new drug = clinical trials etc therefore long process.
Repurposed useful as no need for above but could still be effective.
Trazodone: reduces spongiosis, increases time of survival and not toxic to pancreas.
