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PrionsFirst identified with Spongiform
encephalopathiesCharacteristics of infection:Loss of motor
controlDementiaParalysisEncephalitisWidespread neuronal lossWays of
infection:Infectious (including diet, after surgical procedures,
corneal transplants etc.)Hereditary (autosomal and dominant)
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vacuoleSource: UC Davis School of Veterinary MedicineBrain
Damage from Spongiform Encephalopathy
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Transmissible spongiform encephalopathiesAnimalsBovine
spongiform encephalopathy (BSE)Scrapie in sheep and
goatsTransmissible mink encephalopathyChronic wasting disease of
deer, elkHumansKuruCreutzfeldt-Jacob disease (CJD)Fatal familial
insomnia (FFI)Gerstmann-Straussler syndrome (GSS)TSEs are always
fatal
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Types of TSEsInfectiouse.g., kuru, BSE (mad cow disease),
scrapieSpread by consumption of infected materialIatrogenic spread
(organ transplant, esp. cornea)transfusionSporadic1-2 million
infected worldwide, late in lifeEvidence mounting that some
sporadic TSE is really result of infectionFamilialDue to autosomal
dominant mutation of PrPInherited at least 10-15% of total human
TSE casesEach of these can be transmitted experimentally
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KuruIdentified by epidemiology in New Guinea base on
anthropological research by Robert and Louise Glasse in 1950s1% of
the Fore tribe was afflicted; mostly women, some children, few
adult malesSymptoms: headache, joint pain, then 6-12 weeks later,
difficulty walking, then death usually within 12 months, always
within 2 yearsDisease was of recent origin:
~1910-1920Epidemiological evidence led the Glasses to suggest that
endocannibalism was associated with diseaseThis hypothesis was not
well accepted among medical community
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South Fore
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KuruAustralian government suppressed cannibalism among North
Fore in early 1950sSouth Fore were convinced to discontinue the
practice in 1959Incidence of kuru among North Fore ceased ~ 5 years
before South Fore; no child born since then has died of kuruCarlton
Gadjusek, a medical research scientist with NIH, inoculated chimps
with brain extracts of kuru victims; all chimps died after 50
monthsNo unique antibodies were associated with disease, no virus
particles or aberrant nucleic acids were identifiedGadjusek got
Nobel Prize; Glasses didnt
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ScrapieAn animal model was needed to study TSEsScrapie disease
of sheep had many similarities to kuru in terms of symptomatology
and etiologyCould be transmitted to hamsters and mice, kuru could
notScrapie was used as first good animal model TSE2 month
incubation in rodentsInfectious agent purified 5000 foldNuclease
resistantUV and heat resistantSensitive to protease (only at high
levels) & protein denaturants
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+ = inactivated; - = no change in infectivity
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Glasses research in1950s and 60sIn 1967 Tikvah Alper at
Hammersmith Hospital found particles responsible for transmittable
spongioform encephalitis contained no nucleic acids. Following
characterization of viroids in 1971, many pursued the viroid
hypothesis for prions.In 1982 Stanley Prusiner at UCSF concluded no
NA, first named proteinaceous infectious particles that resist
inactivation by procedures that modify nucleic acids as
PRIONS-received Nobel Prize in 1997.Carlton Gajdusek receive Nobel
Prize in 1976
Major Contributors to the History of Prions
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Bovine spongiform encephalopathy (BSE)mad cow disease
In Britain in the 1970s, hydrocarbon-solvent extraction of meat
and bone meal (MBM) for cattle feed was abandonedIn 1987, BSE
emergedIn 1988, BSE became a reportable disease Epidemiology
suggested a prion disease, and MBM use was abandonedBSE incubation
period is ~5 yearsEstimated that over 1,000,000 cattle were
infectedIn 1989, human consumption of bovine CNS tissue (thought to
have the highest prion concentration) banned based on fears of
transmission to humansIn 1996, a new type of CJD appeared in
Britain and France; young patients (
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Evidence that BSE gave rise to vCJD in humans
Disease was found in younger cohortCourse of vCJD disease was 14
months rather than 4-6 month for CJD, suggesting more distantly
related sourceProteolytic degradation pattern suggests variant CJD
(vCJD) closer to BSE than other CJD strainsMouse inoculations
showed identical reactions with BSE and vCJD, different from
classical CJD; sporadic CJD and all scrapie variants also different
from BSE and vCJDWhen transgenic mice expressing bovine PrPc gene
were inoculated with vCJD or BSE, course of disease was identical
and different from inoculations with CJD or scrapie
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Time course of epidemic BSE in the UK 1986-2000, with dates of
major precautionary interventions. Mammalian ban on meat and bone
meal in March 1996 extended a 1994 ban for farmed food animal
species to include all mammalian species. SBO = specified bovine
offals (brain, spinal cord, thymus, tonsil, spleen, and intestines
from cattle >6 months of age); MBM = meat and bone meal (protein
residue produced by rendering).
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BSE continues to spread to other areas, but has not become
epidemic as it was in Great Britain. It is a major concern because
finding it may result in quarantines against beef from the country
in which it is found.U.S. - 2003
- Cost of Mad Cow Disease3 BSE-infected cows identified in Canada
in May, 2003BSE identified in a cow, originally from Canada, in
Washington state in Dec., 2003; another in Texas in 2005Embargoes
against U.S. and Canadian beef brought immediately by most
importersLoss to U.S. and Canadian beef industries so far due to
embargoes: approximately $10 billionCanada and U.S. test only a
small proportion (
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After BSE was found in Japan in 2001, U.S stopped importing
Japanese beef; Japanese consumption of beef also plunged
THE FIGURES: THE U.S. STORY (EXPORTS)Year Total beef exports to
Japan (in 1,000 USD) Total beef exports to Canada 1998 1,296,265
242,802 1999 1,358,431 225,895 2000 1,449,734 245,003 2001
1,235,392 217,527 2002 831,489 217,690 Jan-May 2003 454,026
111,893
THE FIGURES: THE U.S. STORY (IMPORTS) Year Total beef imports
from Japan Total beef imports from Canada 1997 678 603,022 1998 870
722,828 1999 1,435 918,940 2000 248 962,732 2001 0 1,083,866 2002 0
1,096,238 Source: Foreign Agriculture Service USDA (Figures in U.S.
dollars)
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Different prions affect different parts of the brain
Cerebral cortex When the cerebral cortex is affected, the
symptoms include loss of memory and mental acuity, and sometimes
also visual imparement (CJD).Thalamus Damage to the thalamus may
result in insomnia (FFI). Cerebellum Damage to the cerebellum
results in problems to coordinate body movements and difficulties
to walk (kuru, GSS). Brain stem In the mad cow disease (BSE), the
brain stem is affected.
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Convert PrPc into PrScPrSc has identical primary structure but
different beta structures leading to resistance of protease
cleavage.Brain tissue collects PrSc causing too much protein
accumulation.Distinguished by nerve cell death causing large
vacuoles and plaques in brain tissue
Effect of prions on neural tissue
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How do prions function?PrPScPrPc
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Prion biologyFor a prion (PrPSc) to infect a host, the host must
have a recognizable cellular form (PrPc) of that prionGenerally,
the closer the phylogenetic relationship between the donor host and
the recipient, the greater the chance for infection, and the more
rapidly symptoms occurLevel of accumulation of prion does not
necessarily correspond to level of diseaseMice in which PrPc copy
is knocked out have altered sleep/wake cycles and circadian
rhythm
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Species barrierInfectous dose between species is usually higher
than between animals of the same species (possibly a million fold),
but it is sometimes the same (e.g. between scrapie doses for mink)
When a species has been infected with a TSE of a different species
it can then go on to infect a range of animals that the original
species could not, and with a different dose. When a species has
been infected, it can infect additional animals of the same species
with much lower doses of agent. The histopathology of the disease
in an animal infected from another species is not the same as if it
had been infected from one of the same species. The incubation
period of an animal infected from another species is much longer
than that of an animal from one of the same species.
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Criteria for prion demonstrationTransmissible and associated
with phenotypeReversible curability from cured individual,
phenomenon can arise again because the same event may reoccur in
the same genotypeOverproduction of normal protein increases
frequency of prion formation more normal molecules will be
converted to prion formPhenotype relationship of prion and mutation
of the normal gene for its protein in the host
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Sequence of prion protein
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Cellular trafficking of PrPC and PrPSc. PrPC (yellow dots)
follows the secretory pathway of the cell through the endoplasmic
reticulum (ER) and the Golgi. Mature PrPC is inserted via its GPI
anchor into plasma membrane lipid rafts. The conversion of PrPC to
PrPSc (orange ovals) occurs either on the cell surface or,
following endocytosis, in a cellular compartment such as the
endosome. PrPSc formed at the surface and released into the
extracellular space may cause the plaques seen in TSE diseases such
as human vCJD. The diffuse PrPSc deposits and neuronal vacuolation
common to many sheep scrapie strains may be due to PrPSc formation
in endocytic compartments or to endocytosed surface PrPSc
accumulating inside the cell. Misfolded PrPC (squiggle)
accumulating in the cytosol may also trigger PrPSc formation.
(Inset) Structure of PrPC showing the GPI anchor, the glycan
chains, the copper-binding octapeptide repeats, and the regions
where the helices and loop structure of PrPC (red, blue) may be
converted to the sheets of PrPSc. ERAD, endoplasmic reticulum
associated degradation.Cellular trafficking of PrPC and PrPSc
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Advances in prion controlBSE-resistant cattleBovine PrPc gene
cloned, modified by site-directed mutagenesis to produce
BSE-resistance form Cattle were transformed with modified form of
the gene, targeted to replace natural PrPc geneTransgenic animals
homozygous for mutant gene express mutant copy and are resistant to
BSE, but do not show altered sleep/wake cycles as seen in knockout
miceDepleting neuronal PrPc in prion infection prevents disease and
reverses spongiosisUsing transgenic mice, first demonstration that
prion infection and pathology can be reversed by ceasing expression
of endogenous PrPc copy
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Prions of yeast and fungiYeast and filamentous fungi make great
experimental tools because they are eukaryotes that normally grow
as haploids with small genome sizes and powerful geneticsPrions in
yeast first identified by Wickner as non-Mendelian elements
associated with nitrogen metabolism [URE3], then as a component of
a suppressor tRNA activity [PSI].The first prion in filamentous
fungi was identified in association with heterokaryon (vegetative)
incompatibility in the ascomycete Podospora anserina This is the
only prion identified to date that is not associated with a
diseased state
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Identity of alleles at the het-s locus is required for hyphae of
different Podospora colonies to fuse. However, an encounter of
het-s and het-S colonies will only result in the lethal reaction
that comprises the incompatibility reaction if the Het-s protein is
in its prion form (called [Het-s]).From Wickner, 1999, J.Biol.
Chem. 274: 555The [Het-s] prion results in heterokaryon
incompatibility in the filamentous fungus Podospora anserina.
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Structural model of the SHaPrPC molecule. The model depict
relative sizes of and locations of the Asn-linked oligosaccharides
relative to the published structure of SHaPrP fragments inferred
from NMR spectroscopy. SHaPrPC is shown attached to the plasma
membrane by its GPI anchor to indicate how the range of movement of
the N-terminal half of the molecule might be constrained in vivo.
The putative protein X binding sites are indicated with an X with
lines pointing to the discontinuous epitope on helices C and B with
which it interacts (Kaneko et al., 1997). Adapted from DeArmond et
al. (DeArmond et al., 1999).