Prions

The most recent paper presented to the OPIG journal club from PLOS Pathogens, The Structural Architecture of an Infectious Mammalian Prion Using Electron Cryomicroscopy. But prior to that, I presented a bit of a background to prions in general.

In the 1960s, work was being undertaken by Tikvah Alper and John Stanley Griffith on the nature of a transmissible infection which caused scrapie in sheep. They were interested in how studies of the infection showed it was somehow resistant to ionizing radiation. Infectious elements such as bacteria or viruses were normally destroyed by radiation with the amount of radiation required having a relationship with the size of the infectious particle. However, the infection caused by the scrapie agent appeared to be too small to be caused by even a virus.

In 1982, Stanley Prusiner had successfully purified the infectious agent, discovering that it consisted of a protein. “Because the novel properties of the scrapie agent distinguish it from viruses, plasmids, and viroids, a new term “prion” was proposed to denote a small proteinaceous infectious particle which is resistant to inactivation by most procedures that modify nucleic acids.”
Prusiner’s discovery led to him being awarded the Nobel Prize in 1997.

Whilst there are many different forms of infection, such as parasites, bacteria, fungi and viruses, all of these have a genome. Prions on the other hand are just proteins. Coming in two forms, the naturally occurring cellular (PrPC) and the infectious form PrPSC (Sc referring to scrapie), through an as yet unknown mechanism, PrPSC prions are able to reproduce by forcing beneign PrPC forms into the wrong conformation.  It’s believed that through this conformational change, the following diseases are caused.

  • Bovine Spongiform encephalopathy (mad cow disease)
  • Scrapie in:
    • Sheep
    • Goats
  • Chronic wasting disease in:
    • Deer
    • Elk
    • Moose
    • Reindeer
  • Ostrich spongiform encephalopathy
  • Transmissible mink encephalopathy
  • Feline spongiform  encephalopathy
  • Exotic ungulate encephalopathy
    • Nyala
    • Oryx
    • Greater Kudu
  • Creutzfeldt-Jakob disease in humans

 

 

 

 

 

 

 

 

Whilst it’s commonly accepted that prions are the cause of the above diseases there’s still debate whether the fibrils which are formed when prions misfold are the cause of the disease or caused by it. Due to the nature of prions, attempting to cure these diseases proves extremely difficult. PrPSC is extremely stable and resistant to denaturation by most chemical and physical agents. “Prions have been shown to retain infectivity even following incineration or after being subjected to high autoclave temperatures“. It is thought that chronic wasting disease is normally transmitted through the saliva and faeces of infected animals, however it has been proposed that grass plants bind, retain, uptake, and transport infectious prions, persisting in the environment and causing animals consuming the plants to become infected.

It’s not all doom and gloom however, lichens may long have had a way to degrade prion fibrils. Not just a way, but because it’s apparently no big thing to them, have done so twice. Tests on three different lichens species: Lobaria pulmonaria, Cladonia rangiferina and Parmelia sulcata, indicated at least two logs of reduction, including reduction “following exposure to freshly-collected P. sulcata or an aqueous extract of the lichen”. This has the potential to inactivate the infectious particles persisting in the landscape or be a source for agents to degrade prions.