Prions in soil
Testing the fate of prions in soil takes another step forward.
The misfolded proteins thought to be responsible for brain disorders such as bovine spongiform encephalopathy (BSE), or “mad cow” disease, can stay in soil for a long time, but whether they can still spread the diseases remains a wide-open question. A study published today on ES&T’s Research ASAP website (DOI: 10.1021/es0618189) takes another step toward recovering these so-called prions from soils and adds to previous work on how they might stay sequestered there. The next step is to see what happens when animals are exposed to the recovered prions.
The suite of brain diseases connected to prions includes scrapie in sheep, BSE in cattle, chronic wasting disease in elk and deer, and variant Creutzfeld–Jakob disease in humans. After a massive BSE outbreak in U.K. livestock that peaked in the mid-1990s (which led to fears of transmission to humans), the EU sponsored a research initiative to determine what happens to prions in soils. The ultimate objective is to find out whether the proteins, which come in several forms specific to each disease in different mammals, can cause infection after months or years of burial.
Earlier this year, two groups published data in ES&T (DOI: 10.1021/es060943h and 10.1021/es0516965) examining prions’ behavior in soils. Although the proteins could be eluted from some sandy soil samples from Scotland, clays seemed to trap them. However, the solvents used in this research denatured the proteins, outside researchers note, breaking them down to the point that they were not testable for their infectivity. One team recovered the recombinant version of the protein, which folds into a slightly different form of the infective prion. Whether that form causes disease is not known. Some of that data suggested that microbial activity could break down prions.
Now, researchers led by Robert Somerville of the Institute for Animal Health and Cindy Cooke, who conducted the research while at Imperial College London and is currently at the University of Reading (U.K.), have found that the enzyme proteinase K can readily pull out the form of prions that cause scrapie (known as PrPSc) in mice from sandy soils, while keeping them almost intact. The team also found that prions remained bound up in clay soil samples for several weeks and that they needed proteinase K to pull them out.
The team’s new recovery method is “an important contribution,” considering that “currently there are only a few methods to recover [prions] from soils,” says Joel Pedersen, a soil scientist at the University of Wisconsin, Madison. Pedersen and his co-workers recently published a method in PLoS Pathogens (DOI: 10.1371/journal.ppat.0020032), to recover hamster-adapted scrapie in montmorillonite, a type of clay, but the detergents the team used denatured the proteins. However, the clay-bound prions remain infectious.
The main advantage of the new method, Pedersen notes, is that it “doesn’t appear to require denaturation of the protein to extract it,” but the researchers “haven’t yet proven that the extracted proteins remain infectious.” Other researchers noted that the team did not provide enough quantitative data to compare how much protein was recovered.
The ultimate test, for now, is to take the proteins eluted from the soils and put them into animals and see what happens. But Pedersen says that “another need is more sensitive detection methods that don’t rely on animal bioassays” to see whether infectious prions are present.
