Overview Nature of the infectious particle in TSE TSE strains Role of PrP C in disease Potential...
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![Page 1: Overview Nature of the infectious particle in TSE TSE strains Role of PrP C in disease Potential therapeutic targets Implications for other neurodegenerative.](https://reader035.fdocuments.us/reader035/viewer/2022070305/5513c5ea55034679748b479c/html5/thumbnails/1.jpg)
Overview
• Nature of the infectious particle in TSE
• TSE strains
• Role of PrPC in disease
• Potential therapeutic targets
• Implications for other neurodegenerative diseases
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Prion Diseases: Transmissible Spongiform Encephalopathies
• Fatal neurodegenerative diseases in man and mammals
• Transmissible under natural and experimental conditions
• Lengthy incubation period with no conventional host response
• Characteristic neuropathology with spongiform change in grey matter
• Associated with conversion of PrPC to PrPSc
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Prion diseases of humans and animals
• Scrapie in sheep and goats
• Transmissible mink encephalopathy
• Chronic wasting disease in deer & elk
• Bovine spongiform encephalopathy
• Feline spongiform encephalopathy
• Kuru • Creutzfeldt-Jakob
disease• Gerstmann-Straussler-
Scheinker disease• Fatal familial
insomnia• Variant Creutzfeldt-
Jakob disease
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Protein-only version of the prion hypothesis
• “Prions are transmissible particles that are devoid of nucleic acid and seem to be composed entirely of a modified protein (PrPSc).”
• “The normal, cellular PrP (PrPC) is converted into PrPSc through a post-translational process during which it acquires a high beta-sheet content.”
Prusiner SB, Proc Natl Acad Sci USA 1998;95:13363-83
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Role of PrPC in TSE
• PrPC is required for disease propagation and neuropathology
• PrPC with GPI anchor to cell membrane transduces or potentiates the neurotoxicity of TSE infection
• Tg PrP null mice do not propagate TSE infectivity• Tg mice expressing only anchorless PrPC can
propagate TSE infectivity, but with greatly reduced neuropathology and clinical effects
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Infectious particle in prion diseases
• Nonfibrillar particles between 300-600 kDa (mass equivalent to ~14-28 PrP molecules)
• Other molecular constituents?
• Cofactors for infectivity – sulphated GAG or nucleic acids?
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PrPres Isotype by Western blotTreatment with proteinase K results in N-terminal truncation of PrPres
Distinct isotypes of PrPres characterize different forms of CJD
Isotypes differ in extent of truncation and degree of glycosylation site occupancy
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Multiple conformations of PrPSc?
• “In contrast to pathogens carrying a nucleic acid genome, prions appear to encipher strain-specific properties in the tertiary structure of PrPSc.” (Prusiner)
• Is there evidence for heritable structural diversity in different prion diseases?
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PRNP codon 129 genotype frequencies
MM MV VV
Normal 37% 51% 12%population
Sporadic CJD 71% 15% 14%
vCJD 100% - -
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Idiopathic human prion diseases
Prion disease
PRNP mutation
PRNP codon 129
PrPres isotype
Histological correlate
Reference
Sporadic CJD (Myoclonic, Heidenhain variants)
None
MM, MV
Type 1
Synaptic and coarse granular PrP staining in cortex.
Parchi et al 1999
Sporadic CJD (Ataxic variant)
None
VV
Type 2A
Plaque-like, focal and perineuronal PrP staining.
Parchi et al 1999
Sporadic CJD (Kuru-plaque variant)
None
MV
Type 2A
Amyloid plaques in the cerebellum.
Parchi et al 1999
Sporadic CJD (Sporadic fatal insomnia)
None
MM
Type 2A
Thalamic atrophy. PrP staining faint and variable.
Parchi et al 1999 Pan et al 2001
Sporadic CJD (Cortical variant)
None
MM
Type 2A (Basic glycans)
Cortical perivacuolar PrP staining.
Parchi et al 1999 Pan et al 2001
Sporadic CJD None VV Type 1 Faint synaptic PrP staining.
Parchi et al 1999
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Do different PrPres types replicate with fidelity in vitro?
Soto et al, 2005
When human PrPC is converted to PrPres in a PMCA reaction the product has both the conformation and the glycosylation ratio of the in-put PrPres
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Cellular co-factors & conversion: mammalian RNA
Mammalian brain extracts
contain RNA that stimulate the conversion of PrPC to PrPSc in a modified PMCA reaction
(Deleault et al, Nature 2003;425:717-720)
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Conservation of PrPres isotype following transmission to mice
Inoculum Host Host PRNP PrPres (kDa)
None Human FFI(D178, M129) 19
FFI Mouse Tg(MHu2M) 19
FFI Tg(MHu2M) Mouse Tg(MHu2M) 19
None Human fCJD(E200K) 21
fCJD Mouse Tg(MHu2M) 21
fCJD Tg(MHu2M)
Mouse Tg(MHu2M) 21
Telling et al 1996
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Conservation of targeting following transmission to mice
FFI transmitted to Tg(MHu2M)Prnp0/0 mice
Thalamic pathology
fCJDE200K transmitted to Tg(MHu2M)Prnp0/0 mice
Cortical pathology
Telling et al 1996
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Aspects of PrPSc structure that might encipher strain properties
• Extent of structural re-arrangement (conversion to -sheet) at the N-terminus.
• Presence of methionine or valine at codon 129• Presence or absence of bound divalent cations
(Cu2+)• Extent of of asparagine-linked glycosylation site
occupancy• Composition and complexity of attached glycans
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Pathogenic mechanism
• If we accept the centrality of of the conversion of PrPC to PrPSc in the pathogenic process, then there are in principle three possible alternatives:
– The loss of an essential function of PrPC
– The acquisition of a toxic function by PrPSc
– Production of toxic intermediate or by-product
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Neurodegenerative mechanism
Hope 2000
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Problems with anti-TSE therapy
• Which compound(s) to use?
• What route of delivery to use?
• Is peripheral treatment required?
• How long to treat?
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Approaches to treatment of TSE
• Prevention of PrPC conversion
• Dissolution of PrPSc aggregates
• Enhanced PrPSc clearance
• Neuronal rescue?
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Strategies to prevent PrPC conversion
• Inhibition of expression by RNA interference
• Binding to site(s) for physiological ligands, resulting in PrPC clustering and internalisation from cell surface
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Compounds with in vivo anti-TSE activity
Class/compound ExampleSulphonated dyes Congo red
Sulphated glycans pentosan polysulphate
Cyclic tetrapyrroles porphyrins
Polyene antibiotics amphotericin B
Quinolenes quinacrine
Metal chelators penicillamine
Tetracyclines doxycyline
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Detection of PrPSc in the peripheral tissues in CJD
sCJD
CNSPNSOptic nerveRetinaOlfactory epithelium
vCJD
CNSPNSOptic nerveRetina
AppendixLymph nodePeyers’ patchesTonsilSpleenThymus
Wadsworth et al, (2001), Lancet, 358, pp171-80
Head et al, (2004), American Journal of Pathology, 164, pp143-53
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Probable pattern of tissue infectivity in vCJD
Onset of symptoms
TIME
CNS Infectivity (perhaps 1,000 times higher than LRS)
LRS Infectivity
INF
EC
TIV
ITY
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Neurodegenerative disease and aberrant protein deposition
• Classical neuropathology identifies abnormal histological structures which are diagnostic for particular conditions.
• Nuclear and cytoplasmic inclusion bodies and extracellular amyloid deposits
• Proteinaceous, fibrillar, and rich in -pleated sheet secondary structure
• “Fatal attractions” between abnormally folded forms of specific normal cellular proteins resulting in specific neurodegenerative diseases
• A common feature of Alzheimer disease, Parkinson disease, Huntington disease, amyotrophic lateral sclerosis and prion diseases
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Neurodegenerative diseases associated with abnormal protein conformations
(toxic gain of function)Disease Gene productAlzheimer’s disease APP and A
Creutzfeldt-Jakob disease PrPc and PrPSc
Parkinson’s disease synuclein
Huntingdon’s disease Huntingtin
Machado-Joseph disease Ataxin 3
(SCA 3)
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Neuronal vulnerability to “toxic gain of function”
• Neurones are post-mitotic cells which cannot be replaced (liable to damage by increasing DNA mutations?)
• Unique metabolic demands - some neurones have to maintain an axon over 1m in length
• Functional plasticity• Environment subject to control by many other
structures, including astrocytes and the blood-brain barrier
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Review
• Nature of the infectious particle in TSE
• TSE strains
• Role of PrPC in disease
• Potential therapeutic targets
• Implications for other neurodegenerative diseases