Prions

Prions presentation

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

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

3. Histo r y 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.

4. 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

5. 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

6. P rion 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 PrP C (cellular) PrP-res : Resistant or PrP Sc (scrapie)

7. 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

8. 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.

9. 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

10. 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.

11. PrP-sen featuring more alpha-helices soluble protease-sensitive α-helix rich Normal PrP is soluble in det and readily digested by prot e r ge n ts eases

12. 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

13. 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

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

15. 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.

16. 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.

17. 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”.

18. 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.

19. 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.

20. 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.

21. 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

22. 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

23. 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.

24. 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.

25. 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.

26. 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)

27. 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.

28. 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.

29. 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.

30. 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.

31. 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.

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

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

34. 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

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

36. 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

37. 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.

38. 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.

39. 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

40. 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.

41. 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

42. Ani mal 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

43. 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.

44. Characterized by progressive loss of motor control. Transmissible.

45. 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.

46. 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.

47. 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.

48. 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

49. Pr o gnosis: Death within one year after the onset of symptoms in 90% (further 5% of patients die within the next year