Prions are abnormal infectious protein particles that cause neurodegenerative diseases. They can occur through infection, genetic mutation, or spontaneously. Prions are unique in that they can replicate without nucleic acids by converting normal prion proteins (PrP-sen) into an abnormal form (PrP-res). Accumulation of PrP-res causes neuronal death and symptoms like dementia. Common prion diseases include Creutzfeldt-Jakob disease in humans and mad cow/scrapie in animals. There is no cure for prion diseases and most result in death within a year of symptoms appearing.

1. Prions
By- Sanju Sah
St. Xavier’s college, Maitighar, Kathmandu
Department of Microbiology

3. Introduction
• Prion diseases can occur through
▫ infection (Infectious) ,
▫ as a dominantly inherited genetic disorder
(Familial), or
▫ as a consequence of a spontaneous mutation
(Sporadic).

4. History
• In 1920s, Creutzfeldt and Jakob described first
cases of progressive mental, motor and
neurological deficits in young patients - the
eponym 'Creutzfeldt-Jakob disease’(CJD) was
first used to describe degenerative CNS diseases
• In 1950s, disease known as 'kuru', in Papua New
Guinea discovered; neuropathological
similarity between kuru, CJD.

5. • In 1960s, the term transmissible spongiform
encephalopathy (TSE) applied after discovery of
the transmissible ability of both kuru and CJD
diseases to chimpanzees.
• In 1976, Gajdusek awarded Nobel prize for his
work on 'slow virus' infections theory

6. • In 1980s, the 'protein-only' hypothesis was
developed by Prusiner; he was awarded Nobel
Prize in Medicine in 1997 for his work on
prions; theory remains controversial
• In 1990s, outbreak of BSE in cattle in UK raised
public concern and spawned vigorous research
efforts to understand mechanism underlying
prion diseases

7. Prion
• Prions (PREE-ons) are proteins that are
unique in their ability to reproduce on their own
and become infectious.
• They can occur in two forms
PrP-sen : Sensitive or PrPC (cellular)
PrP-res : Resistant or PrPSc (scrapie)

8. PrP gene structure and expression
• Entire open reading frame (ORF) of all known
mammalian PrP genes resides within a single
exon
• PrP mRNA is constitutively expressed in brain;
highest levels found in the central
nervous system although protein is found in
most tissues, especially cells of the immune
system

9. • Scientists don’t know the exact function of PrP-sen,
but there is evidence that it may be involved in
communication between neurons, cell death, and
controlling sleep patterns.
• Besides:
▫ It may have a role in cell adhesion or signaling
processes
▫ PrP is involved in maintaining myelin formation,
development of T cells and is critical in stem cells
that make blood cells and M cells of the intestine
immune centers.

11. • Normal form of the protein, PrPC, is a
35kDa cell surface glycoprotein that is
anchored to specific areas of the
membrane (caveolae or lipid rafts)by a
glycosyl phosphatidyl inositol (GPI)
lipid linker
• Half-life on the cell surface is 5 h, after
which protein is internalized by a
caveolae-dependent mechanism

12. • The acidic pH of the endosomal
compartment may facilitate the
conformational change
• PrP-sen is produced by normal healthy cells.
• The sen stands for “sensitive” because this
version of the protein is sensitive to being
broken down.

13. • PrP-sen featuring more alpha-helices
• soluble
• protease-sensitive
• α-helix rich
• Normal PrP is soluble in det
and readily digested by prot
ergents
eases

14. • Both PrP-sen and PrP-res are made up of the
exact same string of amino acids, the building
blocks that make up proteins.
• However, the two forms have different shapes

15. • The second type of prion protein, known as PrP-
res(PrP-sc), is the disease-causing form.
• Organisms with it develop spongiform disease.
• “res” stands for “resistant” because this
version of PrP is resistant to being broken
down.
• insoluble filaments
• protease-resistant
• β-sheet rich

16. • Unlike other infectious agents, prions do not
contain genetic material. However, once
they infect an individual, prions can replicate.

17. • Scientists are still working out the details, but
evidence supports the idea that when PrP-sen
comes into contact with PrP-res it is converted to
PrP-res.
• The result is a chain reaction that multiplies
copy after copy of the infectious prion.

18. Prion proteins can exist in multiple structures
called conformers. These different shapes are created
by sigma bonds in their chemical structures that allow
the rotation of parts of the protein.

19. • Because of their abnormal shape, PrP-res
proteins tend to stick to each other.
• Over time, the PrP-res molecules stack up to form
long chains called “amyloid fibers”.

20. • The accumulation of prions in the brain causes
neuronal cells to die and, in some types of TSE, a
type of protein called amyloid accumulates in
plaques, or flat areas, and causes degeneration of
brain tissue.
• Recent research suggests prions disrupt the
normal cell process of protein recycling which
causes a buildup of faulty proteins and causes the
death of the cell.

21. • Amyloid fibers are toxic to cells, and
ultimately kill them.
• Cells called astrocytes crawl through the
brain digesting the dead neurons, leaving
holes where neurons used to be.
• The amyloid fibers remain.

22. • Astrocytes comes into action
• The destruction of neural cells causes tiny
holes in the brain tissue and a sponge-like
appearance under the microscope, thus
giving rise to the term spongiform disease.

23. • As this process progresses, we begin to see
the unique features that characterize all
spongiform diseases:
* Clumps of amyloid fibers
* Holes in the brain
* Large numbers of astrocytes

24. • There are three ways an individual
can end up with a spongiform
disease:
PrP-res
gene that
*Eating tissue infected with
*An inherited mutation in the
codes for PrP-sen
* PrP-res forms spontaneously

26. • Prions cannot be destroyed by boiling, alcohol,
acid, standard autoclaving methods, or
radiation.
• In fact, infected brains that have been sitting in
formaldehyde for decades can still transmit
spongiform disease.

27. Molecular characterization of the PrP
gene.
• PrP gene is highly conserved gene among all vertebrates.
Prion protein gene structure
• PrP is encoded by a single open reading frame (ORF)
encoding 253 amino acids in all known mammalian and avian
PrP genes.
• Alignment of the 44 known PrP sequences shows a striking
degree of conservation between the mammalian sequences,
suggestion the retention of an important function through
evolution.
• However, PrP-null mice are fine.
• Therefore, PrP is not an essential protein.

28. Species barrier
• It is very difficult to establish prion disease in one
species using innoculum from another species ,where
interspecific prion diseases can be established,
incubation times are long.
• However, once prion disease is established in a species,
intraspecific infections are easy to establish, and
incubation times are short.
• Bovine spongiform encephalopathy (BSE) has
been transmitted from cattle to humans to cause a new
variant of Creutzfeldt-Jakob syndrome.

29. Therapeutic approaches for prion
disorders
• Various compounds which bind to PrPSc, including
polyene antibiotics, dextran sulfate, and ß-sheet
breaker peptides have limited effects in animal
models of prion disease. They show a significant
effect only if administered long before clinical onset.
▫ Any ligand that selectively stabilizes the PrPC state will
prevent its rearrangement and block prion replication
(or the production of any toxic intermediate forms of
PrP on the pathway to PrPSc formation)

30. Prions can enter the body through
food and injection
• Injection of prions has occurred during corneal
transplants.
• Prions can, also, enter through blood transfusions,
which demonstrates that blood allows prions entry
into the brain.
• If prions are on the skin or in aerosols, they also end
up in lymph tissue
• It was originally discovered in Kuru, a prion disease
that was self injected by people using sticks for
crushing animal brains to eat. They sharpened their
sticks by stabbing themselves injecting the prion into
their blood.

31. • Prions can enter orally and cross the wall of the GI
tract.
• They survive in gastric acid and intestinal enzymes.
• The prion seems to enter through specialized immune
tissue in the intestine called the Peyer’s patch.
▫ In experiments with animals, inflammation of the
bowel increases the number of prions entering.
▫ M immune cells in the Peyer’s patch take samples of
the material floating by for immune surveillance. M
cells pick up prions from and bring them into the
body. There are other possible mechanisms where
intestinal cells actively transport the proteins into
the cell.

32. Immune System Harbors Prions
• In naturally occurring diseases of many wild
animals, prions multiply in the lymph tissues
before entering the brain.
• Tonsil biopsies can pick up prion disease.
• The spleen is a major place for prions, especially
the stromal region. The spleen appears to be
critical for early multiplication.

33. • Dendritic cells can, also, pick up prions and
transfer them into the lymph system to the
lymph nodes.
• Once in the brain, the higher concentration of
cellular prion protein speeds up the replication
process.
• Prions also may enter lesions or wounds in the
oral cavity and access the Vagus as a pathway to
the brain.

34. • T cells are not responsible for prion multiplication.
• B cells are necessary for prions to enter the brain, but
once they are in the brain they are not dependent on B
cells to spread.
•
• Because prions start as natural proteins they do not
appear to trigger the ordinary innate immune response
that responds to unusual shapes including cancer cells.
• They also don’t trigger the adaptive immunity of T cells
and antibodies.

35. Mammalian Prion Diseases
Human:
• Creutzfeldt-Jakob disease (CJD)
• Gerstmann-Straussler-Scheinker disease (GSS)
• fatal familial insomnia (FFI)
• fatal sporadic insomnia (FSI)
• kuru

36. Animals:
• Scrapie (goats and sheep)
• Bovine spongiform encephalopathy (BSE or Mad
Cow)
• Chronic wasting disease (deer and elk)

38. Creutzfeldt-Jakob disease (CJD)
• The most prevalent of the spongiform diseases
• Occurs spontaneously in 1 out of a million
people
• 10% of cases are inherited mutations in the
PRPN gene
• Usually strikes people age 50 to 75
• Symptoms: dementia, muscle twitching, vision
problem

39. • CJD occurs in three different forms:
▫ sporadic
▫ familial
▫ iatrogenic

40. iCJD (iatrogenic)
 Patients have been infected iatrogenically from
injections of human growth hormone derived
from human pituitary gland extracts,dura mater
graft, cornea transplantation, use of
neurosurgical instruments and EEG depth
electrodes

41. sCJD (spontaneous)
▫ CJD can occur in the absence of a familial history or
infection.
▫ Can be due to a spontaneous mutation on the PrP
gene, or even in the presence of only wild-type genes
• fCJD: (familial)
▫ Dominantly inherited trait, i.e. heterozygous
individuals develop fCJD.
▫ Lybian jews. Had been thought to be due to the
consumption of lightly cooked/raw sheep brain and
eyeballs. Later shown to be due to a specific mutation.

42. Gerstmann-Steussler-Scheinker disease
(GSS)
• The first of the spongiform encephalopathy to be
described as a genetically inherited trait.
• Linkage analysis from GSS families
demonstrated that PrP was the responsible gene.
• GSS may also arise spontaneously.

43. Fatal Familial Insommnia
• FFI: characterized by adults generally over age
50 who present with a progressive sleep disorder
who die within about a year of onset. More than
30 families worldwide have been identified.
• FSI (fatal sporadic insomnia). The sporadic
form of FFI.All cases are inherited mutations in
the PrP gene.Usually strikes people age 36 to
61.Disruption of sleep/wake cycle leads to coma,
then death

44. Kuru
• Affects the Fore people of New Guniea.
• Struck members of the Fore tribe in the 1950s and
1960s
• The Fore people called this sickness kuru, which
means "trembling in fear."
• After intially becoming unable to walk, victims of
kuru lost the ability to swallow or chew.
• Drastic weight loss would inevitably lead to death.
• 1976 Carleton Gajdusek became co-recipient of
the Nobel Prize in Medicine for his "discoveries
concerning new mechanisms for the origin and
dissemination of infectious diseases.

45. • Today we know that kuru is one of several
diseases in humans and animals caused by prion
(PREE-on) proteins.
• Muscle weakness, loss of coordination, tremors,
inappropriate episodes of laughter or crying
• Transmitted by ritual cannibalism as part of
funeral ceremonies

46. Animal
• Scrapie
▫ The classic spongiform encephalopathy of sheep
characterized by progressive loss of motor control
and the propensity of infected animals to
obsessively rub or scrape themselves against
things (fence posts, sides of enclosures, etc) to the
point of scraping off all their hair and rubbing
their skin raw.
▫ Horizontally transmissible through the herd.
▫ There is no evidence of spread to humans

47. Mad Cow disease (Bovine Spongiform
Encephalopathy or BSE)
• Transmissible, neuro-degenerative fatal brain
disease of cattle.
• Long incubation period of 4-5 years
• It is fatal for cattle within weeks to months of its
onset.
• It is transmitted through the consumption of
BSE-contaminated meat and bone meal
supplements in cattle feed.

48. • Characterized by progressive loss of motor
control.
• Transmissible.

49. Route of infection
• Infective sheep and cattle tissue in meat and
bone meal, a component of feed for cattle and
other domestic livestock.
• The traditional includes an extraction step with
organic solvents, which is thought to have
extracted out the prion protein.
• In Great Britian in the early 1980's a new cost
effective method was implemented that omitted
the organic extraction step.

50. • Scientists speculate that the disease has spread
among cattle primarily through "animal recycling" --
the use of bone meal and other ground animal parts
in feeds
• The result was the BSE epidemic of the mid- to late-
80‘s.
• Following the introduction of the ruminant feed ban
in July1988, and the wholesale slaughter of infected
herds, BSE is declining in Great Britain.

51. the human variant of Mad
Cow Disease
Risks to human health
• a new variant of CJD (vCJD) began to appear in
humans 8 years after the first known case of BSE
in cattle.
• Although there have only been 21 confirmed
cases of vCJD to date, the young ages of the
patients and the lack of mutant PrP alleles
suggests that they developed disease via an
infectious route.

52. • GSS is distinct from CJD characterized by
cerebellar ataxia and concomitant motor
problems, dementia is less common (disease
course lasts several years before ultimate death)
• Fatal insomnias present with an untreatable
insomnia and dysautonomia; pathological
changes are characterized by severe selective
atrophy of the thalamus

53. Prognosis:
Death within one year after the onset of symptoms
in 90% (further 5% of patients die within the
next year