On cool summer mornings, Emily Mevers sets out into the forest armed with garden gloves, forceps, and tiny glass specimen jars. The forest floor is carpeted in leaf litter and fallen logs and branches; it’s the perfect habitat for millipedes.
Mevers is a natural products chemist at Virgina Tech. She’s spent her career studying the compounds produced by largely overlooked species, including nudibranchs, fungi, and marine cyanobacteria. Now she’s turned her attention to millipedes, and is studying species from the nearby Virginia woods all the way to the Oregon forests. Millipedes aren’t fast, and they don’t bite or sting; so to protect themselves from becoming an all-too-easy-snack, they produce many different kinds of defensive secretions.
Mevers inspects log after log and eventually comes upon a clump of pale, squirming millipedes and pops a few into one of her collection jars. A clear defensive fluid oozes from glands along the millipedes’ sides, smearing the inside of the jar. Mevers wants to know if these secretions contain compounds that could be useful for humans, potentially as pain medications. After all, animal toxins from cone snail neurotoxin to Gila monster venom are common inspirations for human medicines (1,2).
How did you first become interested in natural products chemistry?
I found this field of chemistry during my undergraduate days. I feel pretty lucky that I found it because there are not many labs around the country that do this kind of research. I went to the University of South Florida and joined Bill Baker’s lab, where I studied the anticancer properties of the small molecules produced by nudibranchs, which are shell-less slugs that live in the ocean. Natural products chemistry merged two things I really enjoyed: the environment in southern Florida and chemistry.
Why are natural products so attractive in terms of drug development?
About 60 percent of all clinically approved drugs can be traced back somehow to a natural product; either it’s the exact natural product, like penicillin, or it mimics a natural product like Taxol, a cancer therapy. Nature is really good at making complex molecules — the kinds of molecules that we can’t easily replicate in the laboratory with synthetic chemistry. They’ve evolved over millennia to have a specific function in nature, so these molecules tend to bind enzymes more specifically and more potently than any other kind of molecule we can find. If we want to try to find a new drug for a disease, a good starting place is looking at the molecules made in nature.
How did you come up with the idea to look at natural products made by millipedes?
I hadn’t really thought about millipedes until I moved to Virginia Tech. There was a researcher named Tappey Jones at the Virginia Military Institute — he just retired actually — and he asked if he could use a piece of analytical equipment that I had in my lab. He told me all about how fascinating millipedes are and about an interesting local millipede species that we should look at.
Nature is really good at making complex molecules — the kinds of molecules that we can’t easily replicate in the laboratory with synthetic chemistry.
- Emily Mevers, Virginia Tech
Another coincidence is that the world expert on millipedes, Paul Marek, also works at Virginia Tech. He was kind enough to take my group out to go find these millipedes. That's probably the most complicated aspect of working with millipedes: learning what millipedes to target and where to find them. Experts have shown us how to do that, and now we're hooked.
Once you know the type of millipede you want to study, how do you find them?
Millipedes eat degrading logs, so we basically go into the forest and just flip over logs until we find them. And we're looking for — as my colleague says — white spaghetti noodles. The white millipedes we collect contrast with the wood pretty well. They're about two centimeters long at their longest, and luckily for us, they don’t move fast. So, when we turn over a log, it’s not like they’re going to run away.
If we go to a new site, it typically takes a couple of hours for us to find them. But if we go back to that site, they're going to be almost always on that same log because millipedes don't move very far.
What kinds of compounds do millipedes produce?
In general, millipedes make three different classes of compounds. Different millipedes make different ones. The more common larger millipedes, make hydrogen cyanide and oxidized aromatics. Those two classes of compounds are really well studied, and we know that they're used to deter predation because the millipedes spray these chemicals at predators.
The less studied compounds are alkaloids, which are much more complex chemical structures. It's not just that they're bigger, but they’re also chiral, meaning that they can be left-handed or right-handed. These molecules are so complex and the millipedes make such large amounts of them, so that told us that the molecules had to do something.
For example, one of the species we’re studying is Ischnocybe plicata. These millipedes make compounds that disorient ants. When we analyzed these millipedes, we found that they produced four new types of alkaloids: ischnocybine A, B, and C and ischnocybinone. When we took the purified alkaloids and treated ants with them, we saw that the ants didn’t want to go anywhere near them. They obsessively cleaned their antennae, and it really seemed like they were bothered by these compounds.
Some alkaloids are well known as neuromodulators. An example would be morphine. Since this class of compounds binds neuroreceptors, we submitted the alkaloids to a psychoactive drug screening program. We screened the compounds against a bunch of different neuroreceptors, and we were surprised by what the ischnocybines, especially ischnocybine A, hit: the sigma one receptor. This is an obscure receptor that, up until recently, was classified as an opioid receptor.
When researchers started sequencing the human genome and understanding how these proteins fold, it became clear that it's not an opioid receptor. It's not related to those at all. We don't know exactly what it does, but scientists have implicated it in modulating pain. So, we think that compounds targeting this receptor might be useful for treating pain, maybe in a nonaddictive way since it's not an opioid receptor.
We don't know the signaling pathway for this receptor, but there are some molecules that bind it in an antagonistic way that are in clinical trials for treating neuropathic pain. The only way of really screening this compound is to put it into an animal study. And the problem with natural products is that we don't get a lot of these molecules. For an animal study, we would need a lot more of these arthropods. We would have to wipe out some of the populations of millipedes, and that’s where we need our synthetic chemistry colleagues to help us out.
What’s next for you?
Right now, we’re looking at other millipedes. My colleague Paul Marek has a millipede library of the species he has found during his career. We are analyzing the defensive secretions from these to identify species that would be worth collecting in the future.
Our longer-term goal is to find the genes that make these molecules in the millipede genome. This compound is an alkaloid, but it’s also a monoterpene. So, we know that it’s probably made out of a terpene and a single amino acid. From the microbial world, we have some idea of what kind of genes would be required to put this together. It probably only requires about four genes, but it’s hard to find those few genes.
We are trying to use RNA sequencing to look at gene expression in the glands compared to the rest of the millipede to see what genes are highly upregulated there. If we can get the glands dissected out, we could use that strategy. Otherwise, there are some reports that small, young millipedes do not make these compounds. So, we're collecting different sizes of millipedes to see if that's true. If we can find a millipede of the same species that doesn’t make the compound, then we can do RNA sequencing on those and compare them to the adults. We’re looking for highly upregulated genes at the time when the compound is being made. Then, maybe one day we can put these genes in a microbe to develop a therapeutic.
This interview has been condensed and edited for clarity.
- Safavi-Hemami, H., Brogan, S.E. & Olivera, B.M. Pain Therapeutics from Cone Snail Venoms: From Ziconotide to Novel Non-Opioid Pathways. J Proteomics 190, 12–20 (2019).
- Triplitt, C. & Chiquette, E. Exenatide: from the Gila monster to the pharmacy. J Am Pharm Assoc (2003) 46, 44–52; quiz 53–55 (2006).