A team of researchers at UC San Francisco, the California Academy of Sciences and Stanford University have uncovered some intriguing clues in the mystery of how some poison birds and frogs evade their own toxins. The answer may lead to a much-sought-after antidote to paralytic shellfish poisoning (PSP) experienced by people eating shellfish gathered after red tides, such as those that have recently plagued coasts of Florida and the Gulf of Mexico.
“We’ve shown that nature didn’t solve this problem with mutations, which is where we often look first,” said Dan Minor Jr., PhD, a professor in the UCSF Cardiovascular Research Institute and one of the senior authors on the study, which appears today in the Journal of General Physiology.
Instead, it appears that there are molecules that can act as ‘toxin sponges,’ mopping up the poisons, and that one of these might provide a treatment for PSP,” he said.
Poison Darts, Tingling Tongues and Red Tides
Minor and his team, which included co-senior author Jack Dumbacher, PhD, Curator of Ornithology and Mammalogy at the California Academy of Sciences, along with Lauren O’Connell, PhD, and J. Du Bois, PhD, from Stanford, studied the Pitohui bird of New Guinea and the golden poison frog of Colombia, Phyllobates terribilis, a fluorescent yellow frog known for its use in making poison darts. The animals get their poison by eating insects that contain it.
Minor and Dumbacher have been discussing research about the toxins since 2011. Dumbacher discovered the Pitohui’s neurotoxicity nearly 30 years ago, when he was holding a bird that bit his hand. To staunch the blood, Dumbacher put his thumb in his mouth and soon felt his tongue and lips tingling. He knew then that they carried poison, likely to protect themselves from predators.
That poison, which the frogs carry as well, is batrachotoxin (BTX), a toxin that is hundreds of times more toxic than cyanide. To get a better picture of toxin resistance in the birds and frogs, Minor and his team compared the activity of BTX to that of saxitoxin (STX), another highly poisonous substance that has been widely studied. STX is produced by microorganisms that live in red tide algal blooms and was once used in now-banned chemical weapons.
There is currently no antidote for either BTX or STX poisoning.
A Mystery Unsolved by Mutations
The two neurotoxins work on the same structures in the cell membrane, tiny tunnels called sodium-channel proteins. These tunnels are usually closed but can open to allow sodium to flow into the cell. STX and many other neurotoxins act like a cork, plugging the sodium channel, but BTX takes the opposite tack, holding the channel open.
“Holding the channel open really jams up the works,” said Minor. “These compounds are so dangerous because these open channels affect electrical signals in the heart and nervous system, causing deadly arrhythmias and paralysis.”
It’s been widely believed that toxic animals have mutations in their channel proteins that keeps them from being poisoned, but Minor and postdoctoral scholar Fayal Abderemane-Ali, PhD, first author of the study, have shown that’s not the case in Pitohui and P. terribliis.
To do so, the team first isolated the sodium channels from the poison birds and frogs and found that they did not contain any protective mutations. In further experiments, the researchers determined that these channels were highly sensitive to both BTX and STX. At the same time, Minor and Abderemane-Ali found that captive-raised poison frogs, who bore these channels but had not eaten the same insects and therefore weren’t poisonous, were totally resistant to both toxins.
“These observations clearly taught us that something was preventing these toxins from reaching their target channels in the first place” Abderemane-Ali said.
The researchers surmised that there must be a mechanism other than mutation at play behind the birds’ and frogs’ autoresistance.
‘Sponges’ Sop Up the Toxins
They sought insight from the protein saxiphilin, which bullfrogs produce, and which can bind to STX. Minor, who was already studying other properties of the protein, wondered whether the poisonous effect of STX could be reversed if saxiphilin was introduced.
His hypothesis turned out to be spot on. The team first confirmed that the sodium channels they had isolated from P. terribilis were poisoned by the STX, after which they were then able to reverse this poisonous effect by introducing saxiphilin.
“We showed that this kind of toxin-binding protein can rescue or protect sodium channels from the effects of STX,” Minor said. “That means that this is a real, viable mechanism for preventing poisoning.”
Dumbacher notes that putting an evolutionary lens on characteristics such as toxin evasion can bring applications for human health into view.
“Every animal has evolved its own solutions to the same problems that humans face,” he said. “When we study adaptations that other animals have come up with, we’re also potentially studying novel solutions that humans can use in our own medicine.”
Minor believes that this collaborative research, bringing together biophysics and evolutionary biology, has shined some light in an area where researchers are not used to looking for explanations, but where Minor is hoping to find more.
“I don’t think saxiphilin is a one-off,” said Minor. “Has nature evolved other systems for binding various toxins? I bet the answer is yes. It just hasn’t been studied yet.”
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