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From the European Society for Clinical Virology 2022 Conference in Manchester UK, Vincent speaks with Richard Knight about prion diseases and the outbreak of bovine spongiform encephalitis that led to cases of variant Creutzfeldt-Jakob disease in humans.
Host: Vincent Racaniello
Guest: Richard Knight
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TWiV revisits chronic wasting disease of cervids and the ability of the prions to infect meadow voles and raccoons, and the suggestion that stochastic assembly of influenza virus particles may play a role in phenotypic diversity.
Hosts: Vincent Racaniello, Dickson Despommier, Rich Condit, and Amy Rosenfeld
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Download TWiV 878 (75 MB .mp3, 124 min)
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by Gertrud U. Rey
Transmissible spongiform encephalopathies (TSEs) include a variety of fatal neurodegenerative diseases caused by infectious proteins called prions. Although prions are not viruses, their ability to self-propagate without a nucleic acid intermediate has always fascinated virologists, causing them to adopt prions into their repertoire of pathogenic agents. Common TSEs comprise scrapie in sheep, bovine spongiform encephalopathy (“mad cow disease”) in cattle, Creutzfeld-Jakob disease in humans, and chronic wasting disease in deer.
Transmissible spongiform encephalopathies (TSEs) are rare, but always fatal, neurodegenerative disorders of humans and other mammals. They are characterized by long incubation periods, spongiform changes in the brain associated with loss of neurons, and the absence of host responses. TSEs are caused by infectious proteins called prions. Insight into how prions cause TSEs comes from the observation that exposure of neurons to prions causes retraction of dendritic spines (link to paper).
Early alterations in the nervous system caused by prions include changes in the synapse such as retraction of dendritic spines, the projections where synaptic contacts occur (illustrated; image credit). Understanding these pathologies has been difficult due to a lack of an appropriate neuronal culture system.
To determine if prions are toxic for neurons, primary neuronal cultures were prepared from mice and grown on layers of astrocytes. Addition of an infected brain homogenate from mice that had been inoculated with the scrapie prion, PrPsc, led within 24 hours to retraction of dendritic spines and a reduction in their number and area. Similar effects on dendritic spines were also observed when purified PrPsc was used.
No effects of brain homogenates were observed using neurons prepared from mice lacking the prion gene prnp. This observation might have been predicted because prion diseases do not occur in mice lacking the prnp gene. However only an N-terminal domain of PrPc (amino acids 23-31) is required for the loss of dendritic spines. It seems likely that this part of PrPc on neurons binds the pathogenic PrPsc form, leading to neuronal loss.
Normal prions (PrPc) are completely digested with the enzyme proteinase K, while the pathogenic prion PrPsc is relatively resistant. Proteinase K treated PrPsc retained the ability to cause retraction of dendritic spines, showing that amino acids 23-90 of the protein are not needed for synaptotoxicity.
Dendritic spines are responsible for excitatory postsynaptic transmission and have roles in learning and memory. Their retraction by pathogenic prions are likely early changes leading to the pathogenic consequences of TSEs. How prions cause spine retractions can now be determined using cultured neurons. It will also be possible to determine if similar mechanisms are involved in dendritic spine loss associated with other neurodegenerative diseases, such as Alzheimer’s, Huntington’s, and Parkinson’s diseases.
Bacteria do not develop transmissible spongiform encephalopathies, but they have been found to produce prions – proteins that can adopt alternative conformations with different functions.
Prion diseases, a frequent topic on this blog, are caused by misfolding of a normal cellular prion protein (illustrated; image copyright ASM Press). Prion proteins are found in other organisms, where the alternative conformation confers a new, non-pathogenic function to the protein. At least 12 different prion proteins have been found in yeast, and they confer the ability to grow more efficiently under certain conditions. Now prions have been discovered in bacteria (link to article).
A search of 60,000 bacterial genomes for proteins with prion-forming domains revealed one in the transcription termination protein Rho from Clostridium botulinum (Cb-Rho). When produced in E. coli, the protein forms amyloid – protein aggregates in the form of fibrils – that are characteristic of prions. A 68 amino acid stretch of Cb-Rho can functionally substitute for the prion-forming domain of a yeast prion-forming protein. This protein, called Sup35, can read stop codons in the prion state, and this phenotype was recapitulated in yeast by the Clostridium prion.
The Cb-Rho prion can convert between prion and non-prion conformations in E. coli. This property was demonstrated by placing a Rho-dependent terminator between a promoter and the lacZ gene, the product of which produces a blue color. In the prion state, Rho has decreased activity, leading to blue cells. In the non-prion state, normal termination leads to pale blue colonies. A mixture of blue and pale blue colonies was observed, showing that Rho exists in the prion and non-prion states.
The prion conformation was also shown to be heritable. Blue colonies always gave rise to blue colonies, while pale blue colonies formed pale blue colonies. The blue colony color lasted for over 120 generations.
The finding of a prion in bacteria indicates that this form of protein-based heredity arose before eukaryotes emerged on Earth. Similar prion-like protein domains have also been found in other phyla of bacteria, suggesting the existence of an important source of epigenetic diversity that can allow bacterial growth under diverse conditions. Exactly how bacterial prions confer new functions will be exciting to discover.
Last time we learned that eukaryotes probably didn’t invent the nucleus. Now we find that prions likely emerged first in bacteria. Did eukaryotes invent anything?
The sages of TWiV explain how chronic wasting disease of cervids could be caused by spontaneous misfolding of prion protein, and the role of the membrane protein Axl in Zika virus entry into cells.
You can find TWiV #426 at microbe.tv/twiv, or listen below.
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