Prions, misshapen proteins suspected of triggering fatal brain diseases, have often baffled scientists. It's not clear how normal proteins become prions, or how prions entering the body--say, via tainted beef--travel to the brain. Now, two independent teams of scientists have come up with possible answers. Although still preliminary, the work could supply clues about how to prevent and treat prion diseases.
In mammals, prions are aberrant forms of a protein called PrP. Some prion diseases occur when healthy PrP spontaneously contorts into its prion form and spreads. But eating meat containing prion PrP apparently can also cause disease.
Adriano Aguzzi, a neuropathologist at the University of Zurich in Switzerland, and his colleagues were curious how ingested prion PrP ends up in the nervous system. His team already knew that prions travel to immune cells in the spleen via the gut lining. But the immune cells that hold prions, follicular dendritic cells, are nowhere near the spleen's nerve cells. Aguzzi wondered if that separation could explain the years-long lag between eating infected meat and becoming seriously ill. To find out, his group closed the gap using genetically altered mice whose follicular dendritic cells touched the nerve cells in the spleen. In these animals, prion disease set in 30 days after the mice received prions, compared to 80 days in controls.
An ocean away, biochemist Surachai Supattapone at Dartmouth Medical School in Hanover, New Hampshire, was seeking the mysterious "factor X"--the molecules that force normal PrP to adopt a prion form. He mixed healthy hamster brain tissue with brain tissue infected with prions. Normally, the infected tissue forces healthy PrP to transform into prion PrP. Supattapone tried adding roughly 20 different molecules, looking for one that slowed this conversion. He hit the jackpot with enzymes that degrade single-stranded mammalian RNA. In high enough doses, the enzymes prevented the conversion altogether. Adding single-stranded RNA, conversely, stimulated the transformation of normal PrP to prion PrP.
Single-stranded RNA "could be the missing cellular factor that's required" for conversion, says Mick Tuite, a molecular biologist at the University of Kent at Canterbury, U.K. However, it's not clear precisely how RNA might interact with PrP. Tuite wonders whether specific RNA molecules are the culprits. If specific molecules are at work, he says, it would be much easier to disable them and prevent prions from forming. As for Aguzzi's study, Supattapone says that "in an elegant way, I think it demonstrates a feasible route of entry" by a prion into the nervous system.