New research on prions, the infectious proteins behind "mad cow" disease and Creutzfeld-Jakob disease in humans, suggests that the ability of prions in one species to infect other species depends on the shape of the toxic threadlike fibers produced by the prion. Two studies on the topic appear in the 8 April issue of the journal Cell.
Although research suggests that prions from one species rarely infect other species, some scientists believe the species barrier was breached when a new version of Creutzfeld-Jakob disease appeared in humans after several recent epidemics of bovine spongiform encephalopathy or "mad cow" disease. Since then, barriers to the transmission of prion diseases between species "have emerged as a major public health issue," according to Eric Jones and Witold Surewicz of Case Western Reserve University.
Prion diseases are caused by misfolded variants of the normal prion protein, which aggregate into fibrous tangles called amyloid fibrils and cause fatal wasting of brain tissue. The abnormally folded protein itself appears to act as an infectious agent, transmitting disease without a DNA or RNA genome such as in a virus. Although disease prions seem to infect normal prions by binding to them and forcing them to take on the abnormal configuration, researchers remain uncertain about the exact molecular details of infection.
Earlier studies identified many "strains" of disease prions across mammalian and yeast species. Researchers thought these strains could be defined by differences in the underlying amino acid sequences of the prions. Under this scenario, disease transmission would be more likely between species with similar prion amino acid sequences.
But a few mysteries stood in the way: Some individuals harbored several different prion strains that caused different disease outcomes, even though all the prions shared the same amino acid sequence. In some cases, a single amino acid change in one species could completely change its ability to infect a previously "off-limits" species, Surewicz and colleagues found.
In a study published last year in the journal Molecular Cell, Surewuicz and colleagues also demonstrated that a "preseeding" process between animals with different prion amino acid sequences could overcome species barriers. For instance, mouse prion fibrils normally infect humans but not hamsters. But when mouse prions were brought into contact with hamster prion amyloid fibrils, a new strain of mouse fibrils emerged with the ability to infect hamsters but not humans. The new mouse strain had the same amino acid sequence as the original mouse strain but completely different infectious capabilities.
With the help of atomic-level microscopic observation of prions in humans, mice, and hamsters, Jones and Surewicz discovered that it is the specific shape of the amyloid fibrils, and not the amino acid sequences, that may allow prions from one species to infect another.
In a second Cell study, Jonathan Weissman and colleagues at the University of California, San Francisco came to the same conclusion in their experiments with yeast. They too discovered that the particular shape of a prion amyloid fibril was the determining factor in whether one species of yeast could infect another yeast species.