Posts Tagged ‘Prions’

How, exactly, do prion proteins cause disease?

Monday, August 5th, 2013

Prion Protein Fibrils via NIAID When prions were first discovered in the 1980s it was immensely controversial whether an isolated protein molecule could act as an infectious agent without any associated nucleic acid, that is, without a genome to encode future generations. I was lucky enough to be able to follow from the sidelines as the prion story slowly unfolded throughout the 1990s. By and large, most of the major questions about prions seem now to have been answered, but one big issue still remains – how exactly do these proteins cause disease?

In contrast to the normally-folded cellular form of the prion protein found in “uninfected” cells, the misfolded disease-causing version of the protein is toxic to brain cells. In a new paper in Nature, Adriano Aguzzi’s group suggests that the prion protein contains a “switch” that controls its toxicity. This switch covers a tiny area on the surface of the protein. If another molecule, for example an antibody, touches this switch, a lethal mechanism is triggered that can lead to very fast cell death (The flexible tail of the prion protein mediates the toxicity of antiprion antibodies. Nature, 31 July 2013 doi: 10.1038/nature12402 - sorry, subscription).

The prion protein consists of two functionally distinct parts: a globular domain, which is tethered to the cell membrane, and a long and unstructured tail. Under normal conditions, the tail is important in order to maintain the functioning of nerve cells. In contrast, in the case of prion infections the pathogenic prion protein interacts with the globular part and the tail causes cell death.

Proteins with similarities to prions also play a role in the pathogenesis of other diseases, such as Creutzfeldt-Jakob disease and possibly some forms of Alzheimer’s, Parkinson’s, Huntington’s, and Lou Gehrig’s disease. Aguzzi et al suggest that prion tail-mediated toxicity could conceivably play a role in these conditions, and that it may be worthwhile screening patients with idiopathic neurodegeneration for disease-causing autoantibodies.

Currently there is no epidemiological evidence for the spreading and/or acceleration of other protein misfolding diseases due to the transfer of misfolded proteins via natural or iatrogenic routes – that is, no evidence that Alzheimer’s, Parkinson’s or Huntington’s disease are infectious in the same way that CJD and other transmissible spongiform encephalopathies (TSEs) are. Such epidemiological evidence may be difficult to interpret for many of these diseases given their multifactorial etiologies and typically long preclinical and clinical phases. However, given the high incidence of diseases such as Alzheimer’s, Parkinson’s, Huntington’s, and Lou Gehrig’s (ALS), it is important to know whether even a small percentage of cases can be initiated by transmission events. Even in the absence of significant human-to-human transmission routes, it is critical to establish whether prion-like propagation of protein misfolding within individuals can be observed and manipulated to alter the course of disease.

Know what triggers prion proteins to cause trouble also provides a possible route of preventing or at least slowing down such diseases. You can bet that a lot of investigators will be looking for triggering antiboes in a wide range of such diseases over the next few years. And if they find them, we can start working on ways to defeat these conditions.

 

Prions and the Potential Transmissibility of Protein Misfolding Diseases. (2013) Annual Review of Microbiology, 67 doi: 10.1146/annurev-micro-092412-155735
Prions, or infectious proteins, represent a major frontier in the study of infectious agents. The prions responsible for mammalian transmissible spongiform encephalopathies (TSEs) are due primarily to infectious self-propagation of misfolded prion proteins. TSE prion structures remain ill-defined, other than being highly structured, self-propagating, and often fibrillar protein multimers with the capacity to seed, or template, the conversion of their normal monomeric precursors into a pathogenic form. Purified TSE prions usually take the form of amyloid fibrils, which are self-seeding ultrastructures common to many serious protein misfolding diseases such as Alzheimer’s, Parkinson’s, Huntington’s and Lou Gehrig’s (amytrophic lateral sclerosis). Indeed, recent reports have now provided evidence of prion-like propagation of several misfolded proteins from cell to cell, if not from tissue to tissue or individual to individual. These findings raise concerns that various protein misfolding diseases might have spreading, prion-like etiologies that contribute to pathogenesis or prevalence.

 

Virology Weekly Newsletter 22.03.2013 – Bumper End of Term Prion Edition!

Friday, March 22nd, 2013

Principles of molecular virology Students taking my virology course at the University of Leicester get a weekly newsletter containing extra links relevant to the lectures. This week we have been looking at sub-viral agents and the class notes are from Principles of Molecular Virology, chapter 10.

 

 

The Importance of Prions. (2013) PLoS Pathogens 9(1): e1003090
While agent host-range and strain properties convinced early researchers of a viral etiology, the once unorthodox postulate that prion transmission occurs by conformational corruption of host-encoded cellular prion protein (PrPC) by a pathogenic isoform (PrPSc) is now widely accepted. Indeed, conformational templating is increasingly understood to be a general mechanism of protein-mediated information transfer and pathogenesis. The high infectivity of prions, their capacity to cause neurodegeneration in genetically tractable animal models, as well as the ability to culture prions in cells, or under cell-free conditions using defined components, provide finely controlled experimental settings in which to elucidate general mechanisms for all diseases involving protein conformational templating, and thus to develop integrated therapeutic approaches.

Unusual infectious agents

Proteins behind mad-cow disease also help brain to develop
Prions are best known as the infectious agents that cause ‘mad cow’ disease and the human versions of it, such as variant Creutzfeldt–Jakob disease. But the proteins also have at least one known useful function, in the cells that insulate nerves, and are suspected to have more. Now researchers have provided the first direct evidence that the proteins play an important role in neurons themselves, lending a hand in forming neuronal connections.

Susan Lindquist: Protein Folding and Prions

Behavior of Prions in the Environment: Implications for Prion Biology. (2013) PLoS Pathogens 9(2): e1003113
The basic parameters of prion environmental interactions are only beginning to be described, and the effect of these interactions on prion transmission and pathogenesis are poorly understood. The interaction of prions in the environment is complex and must include consideration of the route of introduction for prions to the environment as well as the effects of soil properties and prion strain on prion interaction with soil. Once bound to soil, prions do not readily disassociate from the soil particle and remain highly infectious. The implications of these important observations are that prions immobilized to soil may persist at the surface where transmission to a naïve host would be more likely to occur. Strain-specific interactions with the environment may result in preferential selection of strains that have properties that favor environmental persistence and transmission.

Kuru: The Science and The Sorcery

Acquisition of Drug Resistance and Dependence by Prions. (2013) PLoS Pathogens 9(2): e1003158
Prions consist of PrPSc, an aggregated conformer of the host protein PrPC. PrPSc multiplies by catalyzing the conformational conversion of PrPC into a likeness of itself. Prions present as distinct strains that have the same primary amino acid sequence but differ in their conformation. Many distinct mouse-derived prions strains, for example RML, 22L or Me7, have been isolated. Prions can adapt to their environment. We investigated whether propagation of swainsonine-sensitive RML prions in the presence of the drug would yield swainsonine-resistant variants. Propagation of prions in the presence of an inhibitory drug may not only cause the selection of drug-resistant prions but even of stable prion variants that propagate more efficiently in the presence of the drug. These adaptations are most likely due to conformational changes of the abnormal prion protein.

Tis some visitor I muttered, tapping at my chamber door

Thursday, October 18th, 2012

Crow by John K Crows fed on prion-infected brains from mice can transmit these infectious agents in their faeces and may play a role in the geographic spread of diseases caused by prions, such as chronic wasting disease or scrapie. The new research shows that prions can pass through crows’ digestive systems without being destroyed, and may be excreted intact after ingestion by the birds. According to the authors, their results demonstrate a potential role for the common crow in the spread of infectious diseases caused by prions.

Prions are infectious proteins that cause diseases in humans and other animals. Studies so far have suggested that insects, poultry and scavengers like crows may be passive carriers of infectious prions, but this is the first demonstration that prions can retain their ability to cause disease after passing through the avian digestive system. The authors fed crows with brain samples from mice infected with prions, and found that the crows passed infectious prions up to four hours after eating the infected samples. When healthy mice were injected with the infected crow excretions, all the mice showed signs of prion disease. The authors state that their results support the possibility that crows that encounter infected carcasses or consume infected tissue may have the capacity to transport infectious prions to new locations.

 

Prion Remains Infectious after Passage through Digestive System of American Crows (Corvus brachyrhynchos). PLoS ONE 7(10): e45774. doi:10.1371/journal.pone.0045774
Avian scavengers, such as American crows (Corvus brachyrhynchos), have potential to translocate infectious agents (prions) of transmissible spongiform encephalopathy (TSE) diseases including chronic wasting disease, scrapie, and bovine spongiform encephalopathy. We inoculated mice with fecal extracts obtained from 20 American crows that were force-fed material infected with RML-strain scrapie prions. These mice all evinced severe neurological dysfunction 196–231 d postinoculation and tested positive for prion disease. Our results suggest a large proportion of crows that consume prion-positive tissue are capable of passing infectious prions in their feces. Therefore, this common, migratory North American scavenger could play a role in the geographic spread of TSE diseases.

Five Questions on Prion Diseases

Monday, May 21st, 2012

Prion diseases

Nice short review the current knowledge on five issues relevant to prion diseases:

  • How do prions enter the body?
  • How do prions reach the central nervous system?
  • How do prions damage the CNS?
  • Do mammals have an antiprion defense system?
  • How can the prion problem be resolved for good?

 

Five Questions on Prion Diseases. (2012) PLoS Pathog 8(5): e1002651. doi:10.1371/journal.ppat.1002651

The Role of Cofactors in Prion Propagation and Infectivity

Thursday, April 19th, 2012

Prion disease The term “prion” was originally coined by Prusiner to explain the unusual infectious agent in transmissible spongiform encephalopathies (TSEs, also known as prion disease). Now the term has expanded to include a growing list of fungal proteins that stably maintain an atypical self-propagating conformation and epigenetically modify a variety of cellular processes. Although fungal prions and the TSE agent share the capability of maintaining an atypical self-propagating conformation, fungal prions distinctly differ from the TSE agent in several aspects. Thus far, the TSE agent is the only prion that behaves as a bona fide infectious agent, having an infectious cycle, capable of transmitting horizontally (among a community) and causing epidemic outbreaks.

A key concept of the prion hypothesis is that prion is a self-propagating PrP conformer, which elicits the conversion of host-encoded normal PrPC to pathogenic PrPSc. Polyanions, such as RNA molecules and proteoglycans, have been identified as one type of cofactors in the brain homogenate that enhance prion propagation. Lipids are another type of cofactors that promote prion propagation in cell-free conversion assay and in propagating recombinant prions.

 

The Role of Cofactors in Prion Propagation and Infectivity. (2012) PLoS Pathog 8(4): e1002589. doi:10.1371/journal.ppat.1002589

 

Mutation and Selection of Prions

Monday, April 2nd, 2012

Propagation, mutation, and selection of prions in cultured cells
The finding that prions can acquire resistance to drugs has significant implications for drug design. Drugs targeted to PrPSc may have to be administered in combination, as in the case of viruses, in particular HIV. Alternatively, drugs could be targeted to bind and stabilize PrPC or, in view of the finding that ablation of PrPC, at least in animals, is not detrimental to health, to suppress its synthesis. At present no therapeutically useful drugs are available, but deepening insight into the molecular biology of prions may pave the way to novel approaches.

 

Mutation and Selection of Prions. (2012) PLoS Pathog 8(3): e1002582. doi:10.1371/journal.ppat.1002582

 

Biochemical Properties of Highly Neuroinvasive Prion Strains

Saturday, March 10th, 2012

Prion disease Prion diseases are fatal neurodegenerative disorders that are also infectious. Prions are composed of a misfolded, aggregated form of a normal cellular protein that is highly expressed in neurons. Prion-infected individuals show variability in the clinical signs and brain regions that selectively accumulate prions, even within the same species expressing the same prion protein sequence. The basis of these divergent disease phenotypes is unclear, but is thought to be due to different conformations of the misfolded prion protein, known as strains.

Researchers have characterized the neuropathology and biochemical properties of prion strains that efficiently or poorly invade the CNS from their peripheral entry site. Prion strains that efficiently invade the CNS also cause a rapidly terminal disease after an intracerebral exposure. These rapidly lethal strains were unstable when exposed to denaturants or high temperatures, and efficiently accumulated misfolded prion protein over a short incubation period in vivo. These findings indicate that the most invasive, rapidly spreading strains are also the least conformationally stable.

 

Biochemical Properties of Highly Neuroinvasive Prion Strains. (2012) PLoS Pathog 8(2): e1002522. doi:10.1371/journal.ppat.1002522
Infectious prions propagate from peripheral entry sites into the central nervous system (CNS), where they cause progressive neurodegeneration that ultimately leads to death. Yet the pathogenesis of prion disease can vary dramatically depending on the strain, or conformational variant of the aberrantly folded and aggregated protein, PrPSc. Although most prion strains invade the CNS, some prion strains cannot gain entry and do not cause clinical signs of disease. The conformational basis for this remarkable variation in the pathogenesis among strains is unclear. Using mouse-adapted prion strains, here we show that highly neuroinvasive prion strains primarily form diffuse aggregates in brain and are noncongophilic, conformationally unstable in denaturing conditions, and lead to rapidly lethal disease. These neuroinvasive strains efficiently generate PrPSc over short incubation periods. In contrast, the weakly neuroinvasive prion strains form large fibrillary plaques and are stable, congophilic, and inefficiently generate PrPSc over long incubation periods. Overall, these results indicate that the most neuroinvasive prion strains are also the least stable, and support the concept that the efficient replication and unstable nature of the most rapidly converting prions may be a feature linked to their efficient spread into the CNS.

Prions enter stealth mode in the spleen, causing silent infections

Friday, March 9th, 2012

Brains Before they spread to the brain, prions often multiply in the lymphatic system –the group of organs that includes the spleen, lymph nodes, appendix and tonsils. Prions can hide in these tissues, turning individuals into silent carriers even if they never actually develop disease. Worse still, the spleen provides an easy entry-point for prions, allowing them to jump more easily from one species to another.

 

Facilitated cross-species transmission of prions in extraneural tissue. Science (2012) 335(6067): 472-475
Prions are infectious pathogens essentially composed of PrP(Sc), an abnormally folded form of the host-encoded prion protein PrP(C). Constrained steric interactions between PrP(Sc) and PrP(C) are thought to provide prions with species specificity and to control cross-species transmission into other host populations, including humans. We compared the ability of brain and lymphoid tissues from ovine and human PrP transgenic mice to replicate foreign, inefficiently transmitted prions. Lymphoid tissue was consistently more permissive than the brain to prions such as those causing chronic wasting disease and bovine spongiform encephalopathy. Furthermore, when the transmission barrier was overcome through strain shifting in the brain, a distinct agent propagated in the spleen, which retained the ability to infect the original host. Thus, prion cross-species transmission efficacy can exhibit a marked tissue dependence.

See also: Prions enter stealth mode in the spleen, causing silent infections

An overview of human prion diseases

Wednesday, January 11th, 2012

BSE Prion diseases are transmissible, progressive and invariably fatal neurodegenerative conditions associated with misfolding and aggregation of a host-encoded cellular prion protein, PrPC. They have occurred in a wide range of mammalian species including human. Human prion diseases can arise sporadically, be hereditary or be acquired. Sporadic human prion diseases include Cruetzfeldt-Jacob disease (CJD), fatal insomnia and variably protease-sensitive prionopathy. Genetic or familial prion diseases are caused by autosomal dominantly inherited mutations in the gene encoding for PrPC and include familial or genetic CJD, fatal familial insomnia and Gerstmann-Straussler-Scheinker syndrome. Acquired human prion diseases account for only 5% of cases of human prion disease. They include kuru, iatrogenic CJD and a new variant form of CJD that was transmitted to humans from affected cattle via meat consumption especially brain. This review presents information on the epidemiology, etiology, clinical assessment, neuropathology and public health concerns of human prion diseases. The role of the PrP encoding gene (PRNP) in conferring susceptibility to human prion diseases is also discussed.

 

An overview of human prion diseases. (2011) Virology Journal 8: 559 doi:10.1186/1743-422X-8-559