Prions and Viroids

Prions and Viroids

Prions are infectious particles that contain no nucleic acids, and viroids are small plant pathogens that do not encode proteins.

Learning Objectives

Describe prions and viroids and their basic properties

Key Takeaways

Key Points

  • The prion appears to be the first infectious agent found whose transmission is not reliant upon genes made of DNA or RNA.
  • An infectious structural variant of a normal cellular protein called PrP (prion protein) is known to cause spongiform encephalopathies.
  • Prions have been implicated in fatal neurodegenerative diseases, such as kuru in humans and bovine spongiform encephalopathy (BSE) in cattle.
  • Loss of motor control and unusual behaviors are common symptoms of individuals with kuru and BSE; symptoms are usually followed by death.
  • Viroids do not have a capsid or outer envelope and can reproduce only within a host cell.
  • Viroids are not known to cause any human diseases, but they are responsible for crop failures and the loss of millions of dollars in agricultural revenue each year.

Key Terms

  • prion: a self-propagating misfolded conformer of a protein that is responsible for a number of diseases that affect the brain and other neural tissue
  • proteinaceous: of, pertaining to, or consisting of protein
  • viroid: plant pathogens that consist of just a short section of RNA, but without the protein coat typical of viruses

Prions

Prions, so-called because they are proteinaceous, are infectious particles, smaller than viruses, that contain no nucleic acids (neither DNA nor RNA). Historically, the idea of an infectious agent that did not use nucleic acids was considered impossible, but pioneering work by Nobel Prize-winning biologist Stanley Prusiner has convinced the majority of biologists that such agents do indeed exist.

Fatal neurodegenerative diseases, such as kuru in humans and bovine spongiform encephalopathy (BSE) in cattle (commonly known as “mad cow disease”), were shown to be transmitted by prions. The disease was spread by the consumption of meat, nervous tissue, or internal organs between members of the same species. Kuru, native to humans in Papua New Guinea, was spread from human to human via ritualistic cannibalism. BSE, originally detected in the United Kingdom, spread between cattle by the practice of including cattle nervous tissue in feed for other cattle. Individuals with kuru and BSE show symptoms of loss of motor control and unusual behaviors, such as uncontrolled bursts of laughter with kuru, followed by death. Kuru was controlled by inducing the population to abandon its ritualistic cannibalism.

On the other hand, BSE was initially thought to affect only cattle. Cattle that died of BSE had developed lesions or “holes” in the brain, causing the brain tissue to resemble a sponge. Later on in the outbreak, however, it was shown that a similar encephalopathy in humans known as variant Creutzfeldt-Jakob disease (CJD) could be acquired from eating beef from animals with BSE, sparking bans by various countries on the importation of British beef and causing considerable economic damage to the British beef industry. BSE still exists in various areas. Although a rare disease, individuals that acquire CJD are difficult to treat. The disease spreads from human to human by blood, so many countries have banned blood donation from regions associated with BSE.

The cause of spongiform encephalopathies, such as kuru and BSE, is an infectious structural variant of a normal cellular protein called PrP (prion protein). It is this variant that constitutes the prion particle. PrP exists in two forms: PrPc, the normal form of the protein, and PrPsc, the infectious form. Once introduced into the body, the PrPsc contained within the prion binds to PrPc and converts it to PrPsc. This leads to an exponential increase of the PrPsc protein, which aggregates. PrPsc is folded abnormally; the resulting conformation (shape) is directly responsible for the lesions seen in the brains of infected cattle. Thus, although not without some detractors among scientists, the prion appears to be an entirely new form of infectious agent; the first one found whose transmission is not reliant upon genes made of DNA or RNA.

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Example of the formation of a prion: (a) Endogenous normal prion protein (PrPc) is converted into the disease-causing form (PrPsc) when it encounters this variant form of the protein. PrPsc may arise spontaneously in brain tissue, especially if a mutant form of the protein is present, or it may occur via the spread of misfolded prions consumed in food into brain tissue. (b) This prion-infected brain tissue, visualized using light microscopy, shows the vacuoles that give it a spongy texture, typical of transmissible spongiform encephalopathies.

Viroids

Viroids are plant pathogens: small, single-stranded, circular RNA particles that are much simpler than a virus. They do not have a capsid or outer envelope, but, as with viruses, can reproduce only within a host cell. Viroids do not, however, manufacture any proteins. They produce only a single, specific RNA molecule. Human diseases caused by viroids have yet to be identified.

Viroid-infected plants are responsible for crop failures and the loss of millions of dollars in agricultural revenue each year. Some of the plants they infect include potatoes, cucumbers, tomatoes, chrysanthemums, avocados, and coconut palms.

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Potatoes infected by a viroid: These potatoes have been infected by the potato spindle tuber viroid (PSTV). It is typically spread when infected knives are used to cut healthy potatoes, which are then planted.