Prions Flashcards
1
Q
What are prions?
A
- Prons are misfolded forms of cellular prion protein (PrPC),
- PrPC = a normal glycoprotein expressed on the neuronal cell surface and encoded by the PRNP gene
2
Q
What is the primary function of PrPC?
A
- Myelin maintenance
- Other roles:
- Signal transduction
- Apoptosis regulation
- ECM adhesion
- Synapse formation
3
Q
Outline an experiment illustrating the requirement for cellular prion protein (PrPC)
A
- Bueler at al 1993
- Gene knockout experiment
- Gene encoding PrPC (PRNP) was inactivated
- Both the knockout and control groups were inoculated with scrapie prions
- Observed that the KO mice remained symptomless while the wildtype controls all died within 6 months.
- Gene knockout experiment
4
Q
Describe the molecular changes that form a prion
A
- Generated from PrPC by a conformational change
- Converts the native a-helix rich isoform to the beta-sheet rich PrPSc (Sc for scrapie), which is responsible for disease
- Interestingly, Prions are an exception to Anfinsen’s dogma
- Anfinsen’s dogma = aminop acid sequence determines primary, secondary and tertiary structure
5
Q
Describe Creutzfeldt-Jakob disease (CJD)
A
- Creutzfeldt-Jakob disease (CJD) is the most common prion disease
- Initial symptoms
- Abnormalities of memory and behaviour
- Later symptoms
- Rapidly progressive dementia
- Associated with startle myoclonus
- Startle myoclonus = involuntary jerking movements in response to sudden and unexpected stimuli
- Associated with startle myoclonus
- Rapidly progressive dementia
- Inevitably fatal with an average survival time of 7 months after the onset of symptoms;
- Peak incidence = 70 to 80 years.
6
Q
Describe the genetic component of Creutzfeldt-Jakob disease
A
- Single nucleotide polymorphism (SNP) at codon 129 of the PNRP gene that encodes either methionine (ATG) or valine (GTG)
- Predominance of homozygotes amongst suffers for CJD with heterozygosity having a protective effect
- It is thought that the different protein structure alters the probability of conversion for PrPSc to PrPSc but the exact mechanism is unclear
7
Q
Explain how PrPSc mediates protein-mediated transmission
A
- Able to propagate by physical interactions with PrPC,
- PrPC is converted into the misfolded state
- PrPSc acts as a molecular template,
- Provoking conformational changes in PrPC that produce the same abnormal structure
- Because PrP is a normal host protein, PrPSc is not recognised as foreign by the immune system and does not induce an immune response.
8
Q
Explain the evidence suggesting that prions are not transmitted via nucleic acids
A
-
Alper et al in 1967
- Brains of mice infected with scrapie were extracted and made into solutions
- The solutions were subjected to different UV doses including at a wavelength effective at inactivating nucleic acids
- These treated solutions were then injected back into normal mice
- The experimenters observed that all mice developed scrapie regardless of the UV dose
- Suggesting that the infectious agent did not rely on nucleic acid.
9
Q
List examples of prion diseases affecting animals
A
- Scrapie in sheep
- Transmissible mink encephalopathy (TME)
- Chronic wasting disease of elk (CWD)
- Bovine spongiform encephalopathy (BSE) in cattle
10
Q
List examples of prion diseases affecting humans
A
- Familial Creutzfeldt-Jakob disease (fCJD)
- Variant Creutzfeldt-Jakob disease (vCJD)
- Kuru
11
Q
What is the differecne between variant Creutzfeldt-Jakob disease and familial Creutzfeldt-Jakob disease?
A
- Mainly affects young adults
- Behavioural disorders are prominent in the early stage of the disease’
- Neurological syndrome progress more slowly
- vCJD is characterised histologically by fibrillary PrP depositions surrounded by halo of spongiform vesicles as shown by the diagram.
12
Q
Explain the factors that contributed to the Kuru disease endemic. Explain the full pathophysiology of kuru disease.
A
- Cannablism & prion’s long incubation period
- Kuru = transmissible spongiform encephalopath
- Caused by bovine spongiform encephalopathy (BSE) AKA mad cow disease.
- Cattle are believed to be infected by being fed meat-and-bone meal
- Contained the remains of cattle who spontaneously developed the disease or from scrapie-infected sheep products
- Cattle are believed to be infected by being fed meat-and-bone meal
- Spread of vCJD can also occur via blood products or contaminated surgical equipment.
- Caused by bovine spongiform encephalopathy (BSE) AKA mad cow disease.
- Very rare, incurable and fatal neurodegenerative disorder.
- Targets the cerebellum
- Symptoms
- Body tremors
- Random outbursts of laughter
- Gradual loss of co-ordination
- Formely common among the Fore people in Papua New Guinea
- They ritualistically cooked and consumed body parts of their family members following their deaths
- This led to an epidemic from 1957 to 1960
- Resulting in over 200 deaths per year.
- Even though the cannibalistic practice ended in the early 1960s , the disease lingered due to kuru’s long incubation period
- Anywhere from 10 to 50 year
- From 2005 onwards, the deaths from kuru are practically zero
13
Q
How can vCJD be spread? What measures are in place to ensure they are destroyed?
A
- Blood products or contaminated surgical equipment
- Given the resistance of prions to inactivation by treatments that modify nucleic acids such as UV, proteolytic degradation and heat, the normal sterilisation procedures for medical equipment are not protective against prions.
- Infection can therefore be iatrogenic by the introduction of contaminated medical materials into host tissues, such as:
- Surgical tools
- Electrodes for deep brain stimulation
- Supplementary hormone preparations
- However, they can be deactivated in a steam autoclave in the following conditions:
- 132°C
- 21 psi
- 90 minutes
- Although many misfolding diseases are characterised by the protein-level transmission of abnormal structure, only prion diseases are transmissible at the level of the organism
14
Q
Explain how prions can result in neurotoxicity
A
- PrPSc can aggregate into oligomers and eventually amyloid fibrils that commonly surround neuronal bodies and processes.
- This causes neuronal death
- However, prion diseases are not considered amyloidoses because the amyloid deposits do not cause the disease.
- Symptoms
- Rapid progressive dementia
- Cerebellar ataxia
- Akinetic mutism
- Histology
- CNS neuronal loss without replacement by glial cells
-
Spongiform appearance
- Spongiform appearance = s large acellular areas that appear vacuolated in tissue section,
- Commonly occurs in the deep cortical layers, cerebellar cortex or subcortical grey matter
-
Spongiform appearance
- CNS neuronal loss without replacement by glial cells
15
Q
Outline how PrPSc can be detected in biopsies or post-mortem samples
A
- Immunohistochemical detection of PrPSc involves the incubation of brain tissue sections in a solution containing antibodies specific to the protein.
- The antibodies are attached to a peroxidase enzyme,
-
Hydrogen peroxide added to the sample
- Hydrogen peroxide colours the antibodies (attached to peroxidase) and thus the areas containing containing the PrPSc compound
- Areas that have reacted are bleached white
- Non-reactive areas remaisn light blue
- Light blue = colour of hydrogen peroxide
- Light microscopy can then be used to visualise the PrPSc and its distribution.
