Prion disease Flashcards
outline the protein only hypothesis of prion disease
he protein only hypothesis of prion disease argued that disease onset and propagation was not caused via a viral infection but inseadinfection or inheritiance of a pathogenic confromation of the prion protein.
Within this the PRPsc form aggreagates within cell. These are seen as extracellular amyloid fibres. These will induce the maladaptive folding of the PRPc via permissive templating. Within this theory PRPc monomers are incoporated into PRPsc polymers where misfolding is induced. This is then followed by the fission of the polymer in an autocatalytic fashion to produce more template and promote the spreading. Hence, in this way the pathogenic and endogenous forms onl vary on the confromation, with permissive templating of PRPsc being asscoiated with a beta sheet structure, and their incoproration in aggregates.
Basic support for this comes from
- the failiure to identify any viral nucelic acid link to prion disease.
- PRNP KO in mice leads to prion disease resistant mice.
- injection PRPsc strains in the prion cortex can be seen to induce alterations in indogenous PRPc until they appear to have the same conformation as PRPsc injected.
- Mutations in the PRNP gene can cause Prion disease. (familial cases like FFI and GSS)
describe how prion pathology propagates
rion pathology is thought to propagate via permissive templating.
-The pathogenic misfolded structure of PRPsc forms extracellular aggregates in the form of amyloid fibres.
- It is beleived that these polymers take on a a unique structure compared to the endogenous forms. They have two paired fibril each with a underlying repeated helical substructure and they are connected by a unique 8-10nm gap with abnormal topolgy and adhesive properties. This change is believed to fasciliate their infectivity (Terry et al 2019).
These contain that fascilitataes promiscuous binding to PRPc monomers.
-Incorporation of PRPc monomers int PRPsc plymers results in the inudction of misfolding.
-This is followed by the fission of the polymer in a autocatalytic fashion which will the produce more templates to further propagate the spreading of pathology.
templates will vary between strains.
briefly explain how different prion strains are identified in humans
Prion strains have been shown to be proteinase resistant.
- Thus, PRPsc can be differentiated from PRPc by using proteinase K degredation followed by Western blot.
- Western blot in normal vs disease cases will show the harcateristic diglycoslated, monoglycocylated, pure prion forms, with PRPsc showing a predominace in the diglycosylated form.
- Following proteinase degredation we will see a removal of PRPc bands but simply a reduction in molecular weight and thus increase in SDS page mobility of the PRPsc bands.
- Using PGNASE to remove glycosylation can be used to isolate the PRPsc specific protein.
Differentiating between strains:
-Studies have found that the proteinase K degredation western blot of prion strains s distinct. This can be seen when differentiating between the 4 different human PRPsc strains.
Note they can also be differentiated on their histophatholgy.
What is the other way of reffering to prion disease, Why is it termed this?
Prion disease is often reffered to a transmissbale spongiform encephalopathy. This is due to its transmissable nature and reslting spongiform pathology characterised by the vast number of neuronal cells containing autophagic vacuoles.
Outline 7 examples of prion disease in different species.
SCRAPIE- this was primarly seen in sheep and goat and had a lethal and infectious phenotype.
- This was the first form of the disease shown to be transmissbale via inoculation.
- This was demonstrated with the discovery of transmissable Mink encephalopathy which was associated with the use of infected sheep meat in their diet
Chronic wasting disease- Colloquially reffered to a zombie dear disease in the US it is defned the chronic weight loss and reduction in appetite observed.
- There was a lot of worry surrounding this disease as it was estimated a large proportion of american blood donors may have eaten infected venison. However, thus far no prion strains associated with this dieases have been shown to be transmissale to humans.
Bovine sponigform encephalopathy- (vCJD) in humans.
- The most famous from due to it causing Mad cow disease.
- This is a highly transmissable form of the disease across many species. also being asscoiated to feline spongiform encephalopathy in its appearance in zoo animal big cats being fed cow meat.
- The sprread in cow was related to the use of cow bone meal in cow food and thus this has been banned.
- There also maintained fears surrounding this due to the large amount of Bovine cadavers released and consumed by the pubic before the epidemic was realised. Some strain related to BSE linked vCJD have been shown to incubation periods of up to 40 years and thus a pandemic could still be to come.
Kuru-named so due to its discovery in the papa new guines Kuru tribe. this was found predominatly in woman and children.
- The spread was the result of a rtiual cananabalism in which the deceased where eated to allow their soul to pass over.
- The women and children where given the spine and brain to eat as a result of tribe hiercahy and thus consuming these highly infectious regions led to a higher prevalence.
- Characterised by fatal cerebellar ataxia (due to plaque deposition in the cerebellum acting on prukinje neurons) and wide spread amyloid plaques surrounded by spongiform vesicles.
*This is a prime examples of highly infectious regions in this disease. This is much mor wide spead in vCJD but the infectiusnss of the spine and brain here is sared by normal CJD.
- Creutzfeldt Jakob disease- This is the most common human prion disease and can be inherited in a vast array of ways.
- sproadic- onset in the late 60s this is related to sporadic somatic muatatiosn to the PRNP gene of spontaneous converisons of PRPc>PRPsc. (characterised by porgressive cognitive issues in demntia like symtoms that can be see with or without ataxia, related to extra pyramidal effects. 99
- characterised by classic 3 features(GFAP) and often non-specifc neuronal loss. (atrocgliosis , neuronal loss and formation of vacuoles.
- diagnosis with 14-3-3 levels (released from damaged neurons and enhanced in SCJD) in CS can be used to look fo probable and defintie CJD. however, it cannot distingusih between sporadic, iatrogenic CJD and has a high cance of false negatives in disease such herpes complex encephalytis.
- Familial inheritance- This is related to the inheritance of muations in the PRNP gene. (symptoms simmilar to sCJD)Myoclonus is common (spasm/jerk)) in familail formsa
- Iatrogenic- This is the result of mal practice leading the the transfer of pathogenic PRPsc proteins to a healthy brain. This was associated early on with the injection of growth hormones (over 200 patients) from the infected brain and n the use of surgical tools previously used on an infected patient in a healthy patient (This same mechanism has been implacated in the rare spreading of AB AD pathology)
- Aquired- This is related to the onset fo variant CJD through infection via BSE. (Alot more aggressive (average age of onset 29 and averae age of death 30) than other forms with an early age of onset. This has psychological conditions like depression that preceed neurological features like dementia, ataxia and dysarthria and myoclonus.)
- Dmage is most severe in the ocipital and visual cortex. Hippocampal areas are generally sparred whlst spongiform transition is prevalent in the basal galnglia, thalamus (particuarly in PULVINAR)
- Most distingushable feature is the florid plaues, a dense core with pale rradiating outer segements when stained with HE. (flower like( most prominent in the occiptal cortex.
- This also has a much more steryotypes pathology.
- show after 3 months of symptoms onset a Thalamic abnormaility visible with MRI known as the pulvinar sign.
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GSS, Gertsmann straussler schenieker syndrome-
- This shows autosomal dominat inheritance and is related with progressive symptoms of ataxia and pyramidal defects and the eventual presentation of dementia.
- Fatal familial insomnia
- a prime example of the varied targetting of pathology between prion strains.
- This is associated with the severe disrutpion of sleep via dmage to the thalamus in particular. This deprivations evetually results in death.
-Note sporadic prion disease is over 80% of cases.
Why is there still fear lingering over the outbreak of ‘mad cow’ disease?
There also maintained fears surrounding this due to the large amount of Bovine cadavers released and consumed by the pubic before the epidemic was realised. Some strain related to BSE linked vCJD have been shown to incubation periods of up to 40 years and thus a pandemic could still be to come.
Also in Transgenic mice with V129 PRP show a type 5 pathology previously not seen. This could offer a new risk to Varient CJD in the future.
Outline the 4 mechansims link to the inheritance of creutzfelt jakob disease (CJD).
- sproadic- onset in the late 60s this is related to sporadic somatic muatatiosn to the PRNP gene of spontaneous converisons of PRPc>PRPsc.
- Familial inheritance- This is related to the inheritance of muations in the PRNP gene.
- Iatrogenic- This is the result of mal practice leading the the transfer of pathogenic PRPsc proteins to a healthy brain. This was associated early on with the injection of growth hormones from the infected brain and n the use of surgical tools previously used on an infected patient in a healthy patient (This same mechanism has been implacated in the rare spreading of AB AD pathology via the use of contaminated tools, stduy in transgenic mice APP mice shows incolualtion wi wire containg AB fragement could induce the formation of AB plaques. )
- Aquired- This is related to the onset fo variant CJD through infection via BSE.
Besides CJD what other prion diseases can be related to familial inheritance in humans?
GSS, Gertsmann straussler schnieker syndrome-
- This shows autosomal dominat inheritance and is related with progressive symptoms of ataxia and pyramidal defects and the eventual presentation of dementia.
- Fatal familial insomnia
- a prime example of the varied targetting of pathology between prion strains.
- This is associated with the severe disrutpion of sleep via dmage to the thalamus in particular. This deprivations evetually results in death.
Discuss the prion protein structure and its link to disease?
The prion protein is encoded by the PRNP gene.
- This is a glycosylated protein glycosyalted at 2 glycosylation sites with the glycans points inwards.
- Its basal function is currently unknown with Mouse KO studies showing normal behvaioural fucntion and life span.
- The c-terminal in believed to be the signalling region containing 3 alpha helices conneced by a disulphide bridge and 2 beta helices.
- the 3rd alpha helic is linked to a GPI anchor attaching the protein to the cell membrane.
-The N-terminus currently has not been trapped although they know it is octapeptide repeats dense in glycine and thus is thought to modulate ion interactions.
Terry et al 2019
used cryo EM and atomic force microscopy to asssess the structural characteristsic of infectious ex vivo and non infectious invivo prion fibrils.
uninfectious endogenous fibrils have a relatively smooth and basic structure whcih appears to consists of single rods.
Rods in the infectious form show a consist of 2 paired intertwind fibres each with repeating helical substructure with a charcterisitc 8-10nm gap.
- Comapring the artifically adapted rods to those of the Me7 PRPsc strain demonstartaed that their structures where simmilar. indicating that these structural alteration under pin the toxic gain of fucntions of prions.
- This included the abnormally high adhesive properties of the central connecting between the rods. Shwon using AFM to investigate the surface topology.
- This is distinct from the surface that appeared to have no potruding N-linked glycan which is inline with the postulated theory that these glyacans are internalised.
- The unique topagraphy of the charcaterisitc 8-10nm gap between paired fribrisl a seems to be the miost dstingusihable characteristic, it is believed the glyans play an imaortnat role in connected the paired fibres, that likely leds its self to infectivity.
It is likely these changes that promote aggregatation.
-PRNP mutations can cause disease
- Its best links to disease come from the protein only hypothesis.
The protein only hypothesis of prion disease argued that disease onset and propagation was not caused via a viral infection but insead infection or inheritiance of a pathogenic confromation of the prion protein.
Within this the PRPsc form aggreagates within cell. These are seen as extracellular amyloid fibres. These will induce the maladaptive folding of the PRPc via permissive templating. Within this theory PRPc monomers are incoporated into PRPsc polymers where misfolding is induced. This is then followed by the fission of the polymer in an autocatalytic fashion to produce more template and promote the spreading. Hence, in this way the pathogenic and endogenous forms onl vary on the confromation, with permissive templating of PRPsc being asscoiated with a beta sheet structure, and their incoproration in aggregates.
Basic support for this comes from
- the failiure to identify any viral nucelic acid link to prion disease.
- PRNP KO in mice leads to prion disease resistant mice.
- injection PRPsc strains in the prion cortex can be seen to induce alterations in indogenous PRPc until they appear to have the same conformation as PRPsc injected.
- Mutations in the PRNP gene can cause Prion disease. (familial cases like FFI and GSS)
What are the 3 core neuropathological markers of prion disease?
spongifrom change- The neuronal cells are flled with autophagic vacuoles as a result of ongoing autophagy in neurodegeneration give a spogiform appearance.
astrocytosis- The gliosis of glia can be observed, the have infalmed morphology as a result of hypertrophy.
amyolid deposit.- Extracullar amyloid deposits can be observed which are related to the aggeragtion and depostion of mutant prion in amyloid fibres. (This is less common in the sporadic forms and more common in familial forms.)
outline evidence of prion strains. What insight can be taken from this?
This comes from studie sin the SJL mouse model.
- These mice showed expression of distinct neuropathology in terms of the depostion in the cortical layers.
- This could be relaed to 2 distinctive prion strains called MRC1 AND MRC2. Infection in mice could be both distinguished by the layering in the cortex (immunohistochemistry analsysis) and the incubation period (MRC1- 110+-3, MRC2 155+-1.
- These features are stereotypical and distinct
*The existence of strains can expalin the variation in pahthology seen between prion disease. Explaining the specific pion disease like FFI.
These strains are not mutually exclusive and an organism can have many at the same time. It is thought in these situations a dominant strain will control pathology.
*This has also been hyppothesised to explain species boundaries. This being the reson why CWD strains have not yet been shown to be transmissable to humans. Although differing strains can induce conformational changes in others there may be some boundaries in which the PRPc primary sequences are not compatible for templating and transmission.
(Note inheritance between species with different strains has been shown with the changes to the endogenous strain being observed follwing incubation and thus the ransmitted train being reported in studie following death)
- Early on this served as a criticism to the protein only model as it makes the situation much more complex.
- Now it actually provides insight as we can postulate the existence of sevral stables conformations with varied port-translational modifications, and assemblies to medite theis varied pathology.
Discuss the insights from moddeling prion disease in mice?
KO or PRNP goes to resistance to prion disease. shows important of acion on endogeneous PRPc
Successive deletions of the PRPc molecule entally produced a toxic truncated from that drove ataxia and neuronal loss in the cerrebellum. This could be replicated by the ectopic expression of Doppel portein which has a simmilar n-terminal stucture. This phenotye was saved b intorducing PRPc. hence this suggetss that PRPc may have a basal role in neuroprotection.
Although mouse models with KO do not develope prion disease showing it is not an LOF and life span.
- there are some deficits observed. studies have reported mild issues with cognition and the circadian rhythms and in particular reported issues with reduced slow after hypepolarisation in hte hippocampus linked to caclium acitved potassium channels. This could suggest sime detriment to neuronal function.
- these aspects of synaptic dysfunction have been related to some issues with ofactory behvaiour and cognition and thus suggests theat PRPc is important.
Early PRP transgenic models express chimeras of the human PRP gene and mouse gene (TG35). They have shown that these can succesfully propagate both the classcial CJD related type 2 strain to rpoduced diffuse expression and the vCJD linked type 4 strain to produce florid prion plaques in response to innoculation by CJD prions.
-models have shown that KURU, sporadic and iatrogenic CJD strains propagate in simillar ways but vCJD (pathologies vary however) shows a much more distinct propagation.
Other findings pertained to the role of methione or valine 129 residue in the selctive or varied propogation of BSE prions as vCJD type 4 or a novel type taregtting the cingualte cortex.(type 5)
Asante et al 2015
There is a simmilar disovery of a more recent variant at the 127 amino acid confering Glycine or valine. Valine 127 when soley expresse din transgenic mice confered resistace to the propagation of all PRPsc strains.
- They found that it was almost always seen alongside M129 allele. expressing the V127 and m129 PRP in trasngenic mice lines had resistant to classical CJD strain and Kuru showing this new polymorphic region again led conformational selctiveity of propagation conferiing protection. Not resistant to vaian CJD.
What are pottentil therpauetic targets in prions disease?
A pottential target is PRPc misfolding. It is clear in the majority of cases the proression and onset of disease involves the use of endogenous PRPc by the pathogenic form.
Stopping this could be used by bind ith specific antibodies or small molcuels to prevent intercation with PRPsc.
- There are currently limmited downstream effects associated with the sequestrion of PRP.
- This would not require a need for structural information
- should avoid the developement of drug resistance in pathogenic prion strains.
- if we are able to reduce propagation to below the rates of basal clearnace this should be able t clear the infection and cure the infection.
The are currently trialiang an anitbody, PRN 100 in 4 patients and currently it has had no reported toxic side effects.
- The issue is the lack of biomarkers fro this diseases means we cannot track its effect and so it is a case of waiting and seeing what the outcome is.
what are the ongoing uncertainties surrounding vCJD .
The identification of iatrogenic secondary transmission is a worry. There are still worries over the efficacies o methods, like donor screening and decontamination fo surgical instruments in preventing this.
The behvaiour of the BSE strain in intercation with rarer human strains is unknow which could promote new risks in health.
Outline the highly transmissable regions of the body in prion disease?
CJD brain and the spinal cord, eyes and spleen
vCJD- brain, spinal cord, eyes, spleen rectum,thymus, lymph node, tonsils, reminal ileum appendix, adrenal gland
