Bovine spongiform encephalopathy, commonly referred to as mad cow disease, and variant Creutzfeldt-Jakob disease, which (rarely) develops in humans who have eaten diseased tissue, are transmissible spongiform encephalopathies, or TSEs. While most TSE cases arise spontaneously, inherited genetic mutations also can cause these rapidly progressive, fatal and still untreatable neurodegenerative syndromes. The fear caused by the very mention of a case of mad cow disease has immediate effects across the globe and leads to bans on beef imports from the affected country, resulting in enormous socioeconomic consequences.
In the late 1960s, Tikva Alper presented evidence that the infectious agent of TSEs was not a nucleic acid, and John Griffith suggested it was self-propagation of a protein conformer. In 1982, neurologist Stanley Prusiner isolated the TSE infectious agent, finding that its main component was a protein, which he named PrP. He coined the term “prion” to mean “infectious protein.” In 1997, he received the Nobel Prize for this important work. Many aspects, however, were still uncertain and difficult to clarify in experiments involving humans or animals. Surprisingly, the findings that permitted detailed studies and answers came from studies of proteins in yeast by Reed Wickner.
Wickner, who recently wrote a “Reflections” article for The Journal of Biological Chemistry, describes in his article how his background and skills positioned him to undertake these studies. He graduated with a mathematics degree from Cornell University in New York and then went on to study medicine at Georgetown University in Washington, D.C. Upon graduation, he honed his research skills studying enzymes and nucleic acids in postdoctoral fellowships with Herbert Tabor at the National Institutes of Health and Jerry Hurwitz at Albert Einstein College of Medicine. (Tabor was editor of the JBC for four decades and today serves as co-editor.)
In 1973, Wickner returned to the NIH and started working on yeast viruses. During that time, he came across research articles by Francios Lacroute and Michel Aigle that reported that mutations in the yeast protein ure2 affecting the regulation of the enzyme aspartate transcarbamylase had the same phenotype as a spontaneous, nonchromosomal mutant, [URE3], which requires ure2 for propagation. That a nonchromosomal element dependent on a certain gene for its existence could share the same phenotype as mutants of the very same gene got Wickner thinking: It struck him that it was just what one would expect of a prion of the URE2 gene product Ure2p. The lack of mammalianlike prion pathology might have kept others from drawing a similar conclusion, but Wickner had decided to focus on heritable features that would not depend on the particular phenotype produced by the prion.
Using then-new genetic approaches, Wickner showed that the [URE3] phenotype is indeed dependent on the gene ure2, confirming the findings of Lacroute and Aigle. He then went on to show that [URE3] yeast cells grown with low concentrations of guanidinium chloride lose the [URE3] phenotype. However, out of these cured cells, the [URE3] phenotype spontaneously arose again without its introduction from other cells. He also showed that overproduction of Ure2p resulted in a 100- to 200-fold increase in the frequency of [URE3]. This first demonstration of protein-based inheritance involving a protein unrelated to the mammalian prion protein was truly groundbreaking and was published in the journal Science in 1994.
Having broadened the prion concept beyond its restriction to mammals, Wickner went on to show that, at least for Ure2p, amino-acid content — and not amino-acid sequence — determines the ability to form a prion. He also has shown that, like mammalian prions, yeast prions are self-propagating amyloids (filamentous protein multimers) with in-register parallel beta-sheet architecture and has proposed a mechanism for how these prions may template the prion fold of the normal protein.
The highly genetically amenable yeast are well suited for these studies, which would not have been possible in mammals. Yeast have long been used to dissect complex problems in cell biology. They are uniquely amenable to the application of a large number of biochemical and genetic techniques. Their fully sequenced genome is easy to manipulate, they have a short life cycle, and they are inexpensive to grow and maintain. The discovery of the prion system in yeast is a major step forward in prion research, as we now have added to our arsenal the power of yeast genetics and biochemistry. Wickner’s research underscores this, as he was able to confirm the concept that proteins can be infectious, extend this to show that protein conformation can be inherited, and study the mechanisms involved in the generation and propagation of yeast prions. Wickner’s work has not only laid the groundwork for understanding the rare and very debilitating TSEs but also other more prevalent amyloid diseases, such as Alzheimer’s and Parkinson’s.
Wickner’s studies are another example of how basic biochemical studies with no apparent relation to a human disease can lead unpredictably to insights into an important human disease. Wickner was able to carry out these studies only because of his extensive background in yeast genetics and yeast biochemistry.
Karen Muindi (Karen.Muindi@fda.hhs.gov) is a postdoctoral fellow at the U.S. Food and Drug Administration.