1. Overview Nature of the infectious particle in TSE TSE strains
Role of PrPC in disease
Potential therapeutic targets
Implications for other neurodegenerative diseases

2. 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

3. 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

4. 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:

6. 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

7. Infectious particle in prion diseases
Nonfibrillar particles between kDa (mass equivalent to ~14-28 PrP molecules)
Other molecular constituents?
Cofactors for infectivity – sulphated GAG or nucleic acids?

8. PrPres Isotype by Western blot
Treatment 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

9. 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?

10. PRNP codon 129 genotype frequencies
MM MV VV
Normal 37% 51% 12%
population
Sporadic CJD 71% 15% 14%
vCJD 100%

11. Idiopathic human prion diseases

12. Do different PrPres types replicate with fidelity in vitro?
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
Soto et al, 2005

13. 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: )

14. Conservation of PrPres isotype following transmission to mice
Telling et al 1996

15. 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

16. Aspects of PrPSc structure that might encipher strain properties
Extent of structural re-arrangement (conversion to b-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

17. 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

18. Neurodegenerative mechanism
Hope 2000

19. Problems with anti-TSE therapy
Which compound(s) to use?
What route of delivery to use?
Is peripheral treatment required?
How long to treat?

20. Approaches to treatment of TSE
Prevention of PrPC conversion
Dissolution of PrPSc aggregates
Enhanced PrPSc clearance
Neuronal rescue?

21. 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

22. Compounds with in vivo anti-TSE activity
Class/compound Example
Sulphonated dyes Congo red
Sulphated glycans pentosan polysulphate
Cyclic tetrapyrroles porphyrins
Polyene antibiotics amphotericin B
Quinolenes quinacrine
Metal chelators penicillamine
Tetracyclines doxycyline

24. Detection of PrPSc in the peripheral tissues in CJD
vCJD
CNS
PNS
Optic nerve
Retina
Appendix
Lymph node Peyers’ patches
Tonsil
Spleen
Thymus
sCJD
CNS
PNS
Optic nerve
Retina
Olfactory epithelium
Wadsworth et al, (2001), Lancet, 358, pp171-80
Head et al, (2004), American Journal of Pathology, 164, pp143-53
In certain prion diseases PrPSc accumulation has been shown to occur not on in the CNS, but also peripheral tissue
Understanding the peripheral pathogenesis of the different forms of CJD is important for the differential diagnosis of these diseases and provides us with information for assessing the potential risk of iatrogenic spread
vCJD is known to differ from other human prion diseases in the greater extent to which PrPSc can be detected in the peripheral tissues
In contrast, PrPSc is largely confined to the Central Nervous System in sporadic CJD
These two patterns have been thought to be due to the different routes of infection in variant and sCJD.
In the case of vCJD the infection works its way in from the periphery, whereas in sCJD, the disease occurs within the central nervous system and migrates out by centrifugal spread.

26. 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 b-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

27. Neurodegenerative diseases associated with abnormal protein conformations (toxic gain of function)
Disease Gene product
Alzheimer’s disease APP and Ab
Creutzfeldt-Jakob disease PrPc and PrPSc
Parkinson’s disease a synuclein
Huntingdon’s disease Huntingtin
Machado-Joseph disease Ataxin 3
(SCA 3)

28. 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

29. Review Nature of the infectious particle in TSE TSE strains
Role of PrPC in disease
Potential therapeutic targets
Implications for other neurodegenerative diseases