If you had to choose the world's most indestructible biological entity, it would be hard to do better than the prion. It's the Rasputin of biology: cook them, freeze them, disinfect them, pressurize them, irradiate them, douse them with formalin or subject them to protein-cleaving proteases, and yet they live.
Well, not literally live. After all, they're only proteins.
Prions -- infectious misfolded proteins -- have survived the pressure-cooker innards of autoclaves, the stout metal sterilizers that are the backbone of laboratory, hospital, and surgical sterilization. And they have survived for years in the punishing conditions of the outdoors -- wind, cold, rain, snow, ice, heat, and ultraviolet radiation.
These two points should strike fear in the hearts of mammals everywhere, for prions cause incurable fatal neurodegenerative wasting diseases and dementias of the worst imaginable sort -- the kind that swiftly strike down the hale and healthy in their prime.
If you are unfamiliar with prion diseases, that is only because you did not know they were caused by prions. In mammals: scrapie, chronic wasting disease, bovine spongiform encephalopathy, also known as mad cow disease. In humans: kuru, Creutzfeldt-Jakob Disease, and variant Creutzfeldt-Jakob Disease, a tidier name for, well, mad-cow disease.
Once symptoms appear in humans, tremors, convulsions, personality changes, hallucinations, and uncontrollable fits of laughter can precede death, usually within six months or so. Deer and elk slowly emaciate and glaze over mentally under the effects of the chronic wasting disease prion. Sheep with scrapie scrape the fleece from their presumably itchy backsides, and cattle with mad cow stumble around aggressively before succumbing.
Some of these diseases, in addition to their horrific manifestations, also have lurid origin stories. Kuru, for instance, plagued the Fore Tribe of Eastern Papua New Guinea thanks to their habit of consuming deceased members in order to return their life force to the tribe. Women and children were many times more likely to get kuru since the men appropriated the choice cuts, leaving them to eat less desireable bits like brain where prion particles congregate. Eventually authorities intervened to stop the practice.
Mad Cow, as you'll recall, was the result of farmers feeding their cattle ground-up dead cow(called "meat and bone meal"-- recall that cattle are herbivores), in which the prions causing bovine spongiform encephalopathy lurked. These, in turn, may have come from cross-contamination in slaughterhouses that also processed sheep with scrapie. When people in turn ate contaminated dead cow bits, to their horror, they too contracted the fatal wasting disease, and in Britain, at least 165 people died.
There was considerable controversy when the hypothesis that infectious proteins could cause disease was put forth, as the Central Dogma of Biology states that DNA is the unit of heredity and replication, and its bidding is done via RNA and then protein. That aberrant proteins could reproduce themselves, transmit disease and stir up trouble on their own without DNA's help seemed to violate this. When Stanley Pruisner won the Nobel Prize in 1997 for purifying prions, many remained skeptical (though in part because of his sloppiness as an investigator). Even now a few skeptics remain.
Still, the preponderance of the evidence seems to remain with the infectious protein hypothesis. How is it that this could work? Proteins can often change shape. Enzymes -- catalytic proteins -- and other proteins often undergo shape changes when substrates -- the molecules they act upon -- or other cofactors bind to them. These interactions are mediated by various bonds and charges, but to you and me, it looks like simple touch.
Usually these changes are reversible. But sometimes proteins can get stuck in misfolded, extremely stable conformations. What seems to have happened was that the normal prion protein at one point mutated in an individual in a way that changed its shape in an extremely unfortunate manner. Then this protein touched another protein of the same type, inducing a permanent shape change in it too and perpetuating the mistake. Like Pandora's box, once the chain of destruction was initiated, there was no going back.
In infected animals, the more proteins get stuck in the misfolded shape, the more are available to catalyze the reaction. It's exponential. Eventually, the buildup of malfunctioning proteins in sheets and fibrils called amyloid starts killing brain cells. Though the incubation period for prion diseases can be long, once symptoms emerge, the end usually comes nigher rather than later.
There is some controversy over how this happens -- do individual prions simply bump into other individual prions? or do they form long chains or sheets of a substance called amyloid (which you may recall is also a factor in many other neurodegenerative diseases like Alzheimer's) that break frequently and can catalyze reactions at either end? Regardless, the changes induced are permanent, the diseases incurable.
Recall that prions can persist on surgical equipment even after the sterilization of autoclaving. That's BAD. Since Creutzfeldt-Jakob disease in humans can occur spontaneously and the incubation period can be long, people may go into surgery not knowing they are a prion carrier. Scalpels, etc. have been contaminated and then autoclaved, only to spread the prions to helpless victims during subsequent surgery. This nightmare, has, in fact, really happened. New sterilization techniques have been decreed by the World Health Organization to prevent this, but it's a scary thought nonetheless.
In nature, animals have a similar problem. An elk with chronic wasting disease has saliva, urine, and feces full of prions that can linger in soil and contaminate green growth as it bursts forth in spring. When an infected elk dies, these prions are also released into the environment when the animals decay and can similarly hang out in places that elk like to feed. And, as mentioned above, they don't go away. While UV radiation and the extremes of heat and cold can peel paint and crumble newspaper, prions seem to shrug it off. Sheep and deer have indeed been infected after spending time in places contaminated years or decades ago. Since no evidence for a vector like a tick or mosquito exists, the prions seem to be going the same route cold-viruses take on day-care toys and doorknobs: the fomite, or inanimate object vector.
But there is one organism that seems to have found the chink in the prion's formidable armor: the lowly lichen.
Not all of them, mind you. But a few seem to produce a molecule -- likely a serine protease -- or molecules that can take out prions. And they may do it, surprisingly, because fungi seem to get prions too.
Scientists at the U.S. Geological Survey, the University of Wisconsin, Montana State University and the Universidad de Antioquia in Colombia investigated (and published the results in PLoS ONE) what, if any factors could promote prion degredation in the environment by looking at lichens -- fungal/algal/bacterial co-ops which are veritable fonts of chemical and molecular diversity. Lichens produce over 600 "secondary" compounds not essential to their metabolism. They make them for a variety of reasons, including defense from UV, microbes, and herbivory, and as water repellants. Many of these chemicals are responsible for their fantastic colors or fluorescence under UV or surprising color changes in reaction to other chemicals. You can spot a lichenologist in the field by the mini-chemistry labs they haul around for identification.
Since lichens are super-abundant in forest environments (despite the fact hardly any humans notice them), the scientists decided to put a few common lichens in the ring with prions and see who won. For reasons that are unclear to me but may include their abundance in deer and elk habitat, they chose Lobaria pulmonaria, the lungwort, a lichen indicative of pristine forest old-growth northern forests, Cladonia rangiferina, a member of a vastly successful genus common across North America, and Parmelia sulcata, likewise successful in the boreal forests of North America.
What they found was nothing short of stunning. Not only could lichen organic and water extracts degrade prions at least hundred-fold (and sometimes to the point of undetectability), simply incubating the prions in water with an intact lichen could destroy them -- mighty prions, which laugh off the rigors of autoclave and radiation, and I hardly need add, a slew of proteases we ourselves have thrown at them.
The researchers checked other species in the same genera, but these species lacked similar ability. They checked whether the algae the lichen fungi were partnering with were producing the lethal factor, and that seemed unlikely, at least when the algae were in isolation. They examined the effect of pH on the lichens' ability to destroy prion, and found that while P. sulcata's ability to degrade prions was pH sensitive (acidic was better), L. plumonaria's wasn't, suggesting they even have two separate ways of getting the job done -- suggesting that, if the effect is not just due to chance, lichens have figured out how to do this more than once, and it isn't even a big deal.
Further testing suggested it was not one of three common lichen secondary compounds that was responsible, but in fact an enzyme called a serine protease, since only serine protease inhibitors were capable of destroying lichen extracts' prion-fighting powers. Proteins are built of long strings of amino acids, proteases are enzymes that cleave other proteins, and serine proteases have the amino acid serine in their active sites, the seats of catalysis. Why lichen serine proteases can cleave prions where so many other proteases have failed is not known. It's also unknown, the scientists noted, whether some other lichen chemical or protein may be acting as a co-factor that helps the serine protease do its job.
Could lichens provide the same services in nature? Could prions that land on or near lichens whose chemicals may leach out by rainwater reach their ignoble end at last?
No one yet knows.
It's also unknown why lichens might possess this unlikely ability. Yeast -- fungi that have reverted to a single-celled lifestyle -- are known to have prions with amino acid sequences different from the mammalian prions but similar overall sheet-like amyloid structures.They may induce disease sometimes, but in other cases, they may confer an advantage on their hosts by permitting sharing of resources only between individuals that are sufficiently genetically similar.
No one has checked lichens for prions. But since the overall shape of known fungal prions resembles mammalian prions, the researches suggest it's possible lichen proteases could act against fungal prions and mammalion prions alike. Whether putative lichen prions are as destructive as the mammalian forms -- or even if they might be beneficial -- remains in question, but the fact lichens have them suggests prions might be something that lichens are happier without.
You may wonder if lichens could be used to help protect humans from our own prion diseases. This is probably not feasible in surgical environments, both because lichens seem not to achieve complete degradation of prions reliably and because a nuclear option exists: Bleach or sodium hydroxide. Lots of bleach or sodium hydroxide (followed by autoclaving). Bleaching the forest is less feasible. Lichens, however, may be a built-in distributed defense system we didn't even know we had.
Johnson CJ, Bennett JP, Biro SM, Duque-Velasquez JC, Rodriguez CM, Bessen RA, & Rocke TE (2011). Degradation of the disease-associated prion protein by a serine protease from lichens. PloS one, 6 (5) PMID: 21589935