In a breakthrough that could offer new ways to combat mad cow disease and related human brain disorders like vCJD, the infectious prions which trigger these diseases may have been synthesised in a laboratory for the first time.
Researchers have been trying 鈥� unsuccessfully 鈥� to create mammalian prions in a test tube every since Stanley Prusiner, a biochemist at the University of California, San Francisco, first theorised in 1982 that prions were infectious particles composed entirely of protein.
Now Prusiner鈥檚 team reports tantalising evidence that they have been able to build a mammalian prion from scratch. When these synthetic prions were injected into the brains of mice, they triggered a prion disease that could be passed to other animals.
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鈥淭he implications are huge and open up many new ways to study prions and gain new insights about prion disease,鈥� says Giuseppe Legname, a senior scientist in Prusiner鈥檚 lab and one of the lead authors of the study.
鈥淭his is an exciting and fascinating initial experiment,鈥� says Byron Caughey, a prion researcher at the Rocky Mountain Laboratories in Hamilton, Montana. 鈥淏ut we鈥檙e on new ground here. There are details that need to be worked out before it鈥檚 clear this is definitive proof.鈥�
Corrupted shape
According to Prusiner鈥檚 protein-only prion theory, BSE and related diseases occur when a protein called PrP found in healthy brains assumes a corrupted 鈥減rion鈥� shape. The prion can then perform an odd sort of replication by coaxing healthy copies of PrP to flip into the diseased shape.
Folding purified PrP into the prion form in a test tube has been the holy grail of prion science because it seemed like the only way to silence all doubt that protein 鈥� and not some contaminating brain virus, for example 鈥� were actually responsible for BSE. Furthermore, being able to study prions in isolation would give new insights into how PrP folds into a prion and how to prevent or reverse the process to treat prion diseases.
The protein-only theory received a boost in 2000 with the discovery of other proteins from yeast that seem to form prions in a test tube (New 杏吧原创 print edition, 5 August 2000). But despite exhaustive efforts by many labs, no one had managed to perform the same trick with ordinary PrP.
Instead, Legname says his team used a smaller fragment of PrP that was believed to form prions more easily. And rather than inject the synthetic prion into ordinary mice, the researchers used animals that were genetically modified to produce the same PrP fragment at a level 16 times higher than normal, making them more susceptible to prion infection.
The PrP fragment was produced in bacteria, purified and then encouraged to form prion containing fibres known as amyloid.
Rigid tails
Control mice that received a brain injection without the lab-made prions did not develop prion disease after 670 days. But animals that received the synthetic prions started showing the wobbly gait, ungroomed fur and rigid tails that are the clinical signs of rodent prion disease after 380 days.
Extracts from the brains of those diseased animals were injected into normal mice which started getting sick after, on average, only 154 days. That suggested the starting number of synthetic prions was low, but improved after one cycle of replication in a mouse brain, says Legname.
The data suggests that PrP did form a prion in the test tube. But Caughey points out an alternative, though he admits, less likely explanation: the prions weren鈥檛 created in the test tubes, but in brains of mice overproducing the protein.
If that is true, those prions must have replicated so slowly they do not normally kill the mouse, but the additional injection of PrP accelerated their replication. To eliminate this possibility, it will be necessary to show that brain extracts from uninjected mice never trigger prion disease even when injected into a new animal.
Legname says those experiments are already under way. Prusiner鈥檚 team is also busy testing many PrP folding conditions to find ways to make prion formation in the laboratory much more efficient.
Journal reference: Science (vol 305, p 673)
