In class we have spent lots of time discussing prions, how incredibly contagious they are, and how they are virtually indestructible. Unlike every other form of microbe, prions are resistant to radiation, extreme temperatures, metal sterilization techniques, and autoclaving. This means that if certain tools are used in surgery on a person with a prion-related disease, the tools remain infected despite all sterilization attempts. Scary! Additionally, if an infected animal dies in an environment, the environment will remain infected for decades to come!
So why haven’t prions created a wide-spread pandemic if they are so contagious and indestructible? The answer lies in lichens, forest fungi that cover tree bark and much of the forest floor. There are hundreds of varieties and several produce a molecule — likely a serine protease — or molecules that can take out prions.
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.
Since lichens are super-abundant in forest environments, the scientists decided to put a few common lichens in the ring with prions and see who won. 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.
Further research 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.
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.