Misfolded proteins called prions are thought to trigger deadly brain illnesses including mad cow disease. Beyond that, they're pretty much a mystery to researchers. No one knows what makes a normal protein become a prion, for example, or how to prevent the transformation. Now, researchers have found by chance that fusing an antibody molecule to a protein with prion potential stops that misfolding.
Biologists have identified a few proteins that can misfold and form prions in various species. Those prions, in turn, "infect" their neighbors, forcing similar misfolding. In mammals, just one protein, called PrP, is known to become a prion. But it has sowed panic in European countries, where PrP's prion form is believed to have spread mad cow disease to people, killing more than 100. No treatment currently exists.
Adriano Aguzzi, a neuropathologist at the University of Zurich in Switzerland, and his colleagues, wanted to trace the way normal protein converts into its prion form in mice. To do that, they altered the gene that produces PrP, coaxing it to generate a new protein that's fused to a human antibody. This, Aguzzi's group reasoned, would make the protein easier to track.
Mice injected with the prion version of PrP usually die quickly of scrapie, a prion disease. But the researchers were taken aback when the genetically modified animals showed no ill effects at all. Studying the animals' brains, Aguzzi's team found no trace of prions. A second test showed that the protein can slow the disease's progression: Mice designed to express both the normal and modified protein, and then injected with prions, developed scrapie 80 to 120 days later than controls, and some stayed alive longer than expected, the group writes in the 4 April issue of Cell. The researchers are still trying to understand why the fused protein protected the animals and are also testing whether inoculating mice with this protein will have the same effect. One key concern is whether the fused protein can cross the blood-brain barrier, which protects the brain from toxins but also makes drug delivery difficult.
"This is beginning to provide a model system to really understand what's going on" in animals, says Mick Tuite, a molecular biologist at the University of Kent in Canterbury, United Kingdom. It's not clear, he says, which portion of the antibody molecule upends PrP's infectivity. And although this technique may be exploitable as a therapy, he says, "it's not a cure. It stems the disease."