Toxic? These Animals Don't Care

Imagine, you’re a toxic toad hanging around South America. No other animals are gonna mess with you, right? After all, you’re ~toxic~! So if anyone tries to eat you, they’ll be exposed to something called a cardiotonic steroid — and may die of a heart attack. Well, unfortunately, for you, some animals have developed adaptations to these toxic steroids. Evolutionary biologist Shabnam Mohammadi https://www.mohammadi-lab.com/people.html has spent her career studying how these adaptations work — and says even humans have used these toxins to their advantage since ancient Egypt. So today on Short Wave, we get a little… toxic (cue Brittney Spears). Host Regina G. Barber https://www.npr.org/people/1082526815/regina-g-barber talks to Shabnam about how some predators can get away with eating toxic prey. Curious about biology? Email us at [email protected]. Listen to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ at plus.npr.org/shortwave http://plus.npr.org/shortwave. See pcm.adswizz.com https://pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences. NPR Privacy Policy https://www.npr.org/about-npr/179878450/privacy-policy

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Speaker 00

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• 0:16Speaker 01

  You're listening to Short Wave from NPR.

• 0:22Regina Barber?

  Imagine you're a toxic toad hanging around South America. When you finally get your big break, you and your family are going abroad, to Japan, as pest control. No other animals are going to mess with you, right? After all, you're toxic. If anyone tries to eat you, they'll be exposed to something called a cardiotonic steroid, and they may die of a heart attack.

• 0:45Speaker 03

  And when they're ingested and they reach the bloodstream, and from the bloodstream they reach cells, they bind to and disable one particular protein, and this protein is the sodium potassium pump.

• 1:00Speaker 03

  It plays a role in almost every single physiological system: blood pressure regulation, cardiac contractility, brain function.

• 1:11Regina Barber?

  Evolutionary biologist Shabnam Momadi says there are hundreds of different kinds of these toxins. But wait, that bird over there just ate your cousin, and it doesn't seem to be getting sick at all. What the heck is going on? Unfortunately for toads, some animals have developed adaptations to toxic steroids. And Shabnam, who goes by Shab, has spent her whole career studying how animals can eat foods that are laced with these poisons without dying.

• 1:39Speaker 03

  What is their adaptation? How does it work?

• 1:41Regina Barber?

  Humans have used this toxin to their advantage throughout time. The first written evidence goes all the way back to ancient Egypt.

• 1:48Speaker 03

  There's a papyrus known as the Eber Papyrus. They documented the use of plants that contain cardiotonic steroids to treat people with heart failure or heart problems. And to this day, we still do that, actually. Some versions of cardiotonic steroids are prescribed for treating congestive heart failure.

• 2:12Regina Barber?

  Today on the show, toxicity. It can feel ubiquitous in this world, and yet some predators have learned to overcome it. How do they do it? And what does all of this have to do with Vincent Van Gogh? I'm Regina Barber, and you're listening to Shore Wave, the science podcast from NPR.

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• 4:04Regina Barber?

  Okay, Shab, so we heard one way this toxin, cardiotonic steroids, interacts with us humans is like with our hearts. Now, I want to talk about another way it might interact with us, and that's our microbes, our gut microbes. How are gut microbes interacting with this toxin?

• 4:20Speaker 03

  Okay, the story actually starts with, I think it was clinical trials. They were giving patients cardiotonic steroid doses. Some people got placebo, some people got the cardiotonic steroid. And they found that some individuals excreted different amounts of the cardiotonic steroid that they were given. And also, different individuals had different levels of cardiotonic steroid in their blood after they were given a dose. So they were trying to figure out why are different amounts of this toxin ending up in different people's blood?

• 5:02Speaker 03

  And there was a group from UC San Francisco led by Peter Turnbaugh, I believe it was around 2013, 2015. They ran a study where they basically discovered a microbial source of resistance to cardiotonic steroids. And what they found was that actually there is a bacteria in our gut. Not everybody has it, and some people have more of it than others. This bacteria can actually metabolize cardiotonic steroids. So when you ingest them, they go in your gut, and these bacteria break it down so that

• 5:44Speaker 03

  less ends up getting absorbed in your blood and circulating through your body.

• 5:48Regina Barber?

  Wow.

• 5:49Speaker 03

  And in that sense, the bacteria are providing resistance for you.

• 5:53Regina Barber?

  Wow. Like somehow, humans can be maybe a little resistant to this toxin?

• 5:58Speaker 03

  Yeah, yeah, yeah. Thanks to our gut bacteria. As long as you have a good, healthy gut flora.

• 6:04Regina Barber?

  Wow. The famous painter Vincent Mago somehow is like related to these toxic plants. Can you tell me about that?

• 6:11Speaker 03

  Yeah, that's a fun theory. One of his famous paintings, it's a painting, I don't remember the name of it. It's a painting of his doctor, and he's leaning on the table with his elbow on the table, and next to him is a plant. And this plant is the foxglove. And foxglove is a plant that produces the cardiotonic steroid digoxin, which is actually what is most commonly used today for heart failure therapy.

• 6:40Regina Barber?

  Oh, wow.

• 6:41Speaker 03

  So there's this theory that Vincent van Gogh was taking foxglove recreationally. Therefore, he was ingesting digoxin. And digoxin has tons of side effects. There are common ones, there are rare ones. And one of the rare ones, or a few of the rare ones, are seeing bright halos around lights. And in particular, yellow.

• 7:06Regina Barber?

  Like Starry Night, like the painting.

• 7:09Speaker 03

  Exactly, yeah.

• 7:09Regina Barber?

  Wow.

• 7:11Speaker 03

  It fits really well to his painting style. So there's a theory that he, maybe the reason why he painted the way that he did was because he was recreationally consuming cardiotonic steroids.

• 7:21Regina Barber?

  Wow, I like it. Let's go from plants to animals besides humans, different kinds of animals. How are these animals, like the toads you were talking about, using this toxin for their own defense?

• 7:36Speaker 03

  They synthesize this toxin from cholesterol. And then they concentrate it in these glands on their skin. They have two on the back of their necks. They're the parotid glands, and that's where most of it is concentrated. But they also have a lot of other smaller glands throughout their skin, which also contain cardiotonic steroids. And essentially, when an animal bites into a toad, these glands release the toxins and then the animal ingests it.

• 8:04Regina Barber?

  Wow. And is it only plants and toads? Or are there other things that have this toxin at high levels that you're talking about?

• 8:16Speaker 03

  Yeah, you find it also in a lot of insects. So fireflies, for example, produce it. Monarch butterflies are a famous example. The caterpillars feed on milkweed, and they then take the cardiotonic steroids from the milkweed and store it in their own body. This process is known as sequestration. And the European hedgehog, they also chew on toad skin. They take the cardiotonic steroids and they lick it onto their spines. So they're producing their own chemical defense. This is a mechanical form of sequestration. And there's a rat in Africa, the African crested rat, that does the exact same thing

• 8:58Speaker 03

  with plant-derived cardiotonic steroids. And they're actually, they've taken it to the next level. Their hairs have these hollow spaces and those spaces can wick the toxin into the hair. So they can actually, in a sense, store it in their hair.

• 9:16Regina Barber?

  That is so cool. Okay, so this is what all of these animals are doing to defend themselves against these predators. But some of these predators can eat these toxins without getting sick? How do they do that?

• 9:27Speaker 03

  Yeah, some animals have developed mechanical resistance. This you see often with birds where they'll just eat the insides of the toad and leave the skin out. They know that the skin is toxic.

• 9:40Regina Barber?

  Oh, wow.

• 9:41Speaker 03

  There's actually a record of a snake in Asia that can also do this. So this is really remarkable because snakes don't have any digits, any hands, feet, nothing. But they do it with their mouths somehow. They're able to just bite around in the stomach area and get that out and leave the skin behind.

• 10:03Speaker 03

  The evolution of resistance to cardiotonic steroids is an adaptation that has happened repeatedly across the world, across the animal kingdom.

• 10:13Regina Barber?

  This is called convergent evolution, right?

• 10:16Speaker 03

  Yeah, convergent evolution in a broad sense refers to when you have multiple organisms that are not related to each other, which have all evolved the same adaptive solution to a problem independently. And the way that most predators of cardiotonic steroid-defended animals and herbivores of cardiotonic steroid-defended plants resist these toxins is through just a handful of mutations on the target protein of this toxin, which I mentioned earlier was the sodium potassium pump.

• 10:57Speaker 03

  And the really cool thing is these mutations, they pretty much happen at the same site on the protein over and over again. So all of these animals, snakes, birds, frogs, mammals, have evolved resistance to cardiotonic steroids by one or two mutations at the same place on the sodium potassium pump. And they do it through different mutations, but it's always at the same site.

• 11:28Regina Barber?

  What is your labs, like, the big question you want to answer? Like, what is your main focus?

• 11:34Speaker 03

  The big cosmic question that we're trying to answer is how adaptation happens as a process. What are the dynamics of the adaptive process? And the approach that we take is to actually reconstruct evolution in vitro, in the lab. And we do this using statistics. So we're able to statistically reconstruct ancestral steps, you know, hundreds of millions of years ago. And we do this at the gene level. So we can reconstruct the gene for this protein, 60 million years in the past, 100 million years in the past, so on and so forth. And then we resurrect that in the lab at the protein level.

• 12:19Speaker 03

  And then what we do is we take these modern resistance mutations and we move them back in time onto each step to see how the effects change as they go back in time. And this sort of tells us at what point this adaptation was possible.

• 12:35Regina Barber?

  Yeah, I mean, we've talked about cardiac arrest help. Like, what are the, what are like the human applications to studying cardiotonic steroids?

• 12:47Speaker 03

  Oh, OK. A collaborator of mine, Gustavo Blanco, from the University of Kansas Medical School.

• 12:57Speaker 03

  Is one of the groups that's working on developing male birth control. And, long story short, mammals have-- so this protein that these toxins attack and disable, the sodium potassium pump, exists in four copies. So there are four versions of this protein in our bodies. And they exist in different tissues. For example, one copy is in our heart, one copy is in our brain, so on and so forth. And there's one copy that is present only in mammalian sperm. This is copy number four. And what they have found is when they silence this copy, or knock it out,

• 13:40Speaker 03

  then the sperms are not able to move anymore. So this protein is vital for sperm mobility.

• 13:47Regina Barber?

  Oh, wow.

• 13:48Speaker 03

  And what disables sodium potassium pump? Cardiotonic steroids. So they're looking to develop a cardiotonic steroid compound that just targets this copy number four. If they can disable only copy number four, then they can disable sperm.

• 14:05Regina Barber?

  Wow. Shab, we went from, you know, predators eating toxic animals to Vincent Van Gogh to male contraception. I loved this. Thank you so much for talking to me today.

• 14:21Speaker 03

  It was really fun to talk about. Yeah, thank you.

• 14:27Regina Barber?

  Shortwavers, thank you for listening. If you want to help us out, follow us on the NPR app or whatever podcasting app you use. This episode was produced by Rachel Carlson and edited by our showrunner, Rebecca Ramirez. Tyler Jones checked the facts. Kwasi Lee was the audio engineer. Beth Donovan is our senior director and Colin Campbell is our senior vice president of podcasting strategy. I'm Regina Barber. Thank you for listening to Short Wave from NPR.