The devil is in the details: how poison-dart frogs avoid poisoning themselves

On a frog-catching expedition to the jungles of Ecuador, Aurora Alvarez-Buylla, a Ph. D candidate from Dr. Lauren O’Connell’s laboratory at Stanford, recounts the astounding expertise of wildlife scientist Dr. Elicio Tapia, the “frog whisperer”. 

“We drove a dirt road in an old, creaky Toyota. I could not hear a thing.” Tapia, one hand on the wheel and his head out the window, remained alert. The truck stops, the whisperer exclaims: “There’s a frog.” 

Silence, then: ‘bwap-bwap-bwap’, the characteristic mating call of Oophaga sylvatica, a 1.5 inch-long poison-dart frog with yellow-marbled skin.

Oophaga sylvatica, known by locals as ‘el Diablito’ (‘the little devil’), brands eye-snatching colors that mean BEWARE; its colorful skin is an honest signal to the frog’s predators–Diablito’s way of saying: “if you munch me, you’ll regret it!”

Accumulated in el Diablito’s skin are toxic alkaloids; small molecules obtained from the ant and mite-rich diet poison-dart frogs consume. Inside the body of its enemies, these toxins disrupt the nervous system and muscle cells. The consequences can be as mild as a nasty taste, enough to make the predator go “yuck!” and spit Diablito out, or as potent as to cause paralysis or cardiac arrest. 

Little is known about how poison-dart frogs achieve and survive the accumulation of potentially lethal chemicals. Alvarez-Buylla thinks that poison-dart frogs may achieve this impressive feat with the help of transporter proteins in their blood, which may hang on to the toxins and prevent them from wreaking havoc until they are shuttled to the skin.

Alvarez-Buylla and her colleagues devised an elegant way to figure this out: they ran experiments to see if proteins in the frog’s blood were interacting with pumiliotoxin (an alkaloid often found in Diablito’s skin) and a mimic of this molecule outfitted with a light-sensitive dye. If proteins bound to the mimic, it lit up; but if the proteins preferentially bound to pumiliotoxin or other alkaloids, then the interaction was invisible. Using this comparison, the researchers were then able to identify the most prevalent protein binding to both the mimic and the alkaloid, and appropriately baptized it alkaloid binding globulin (ABG). 

With a clear protein candidate, Alvarez-Buylla and colleagues found that ABG is not only present in the blood, but also in the gut, liver, and, most importantly, the skin, highlighting its critical role in toxin transport throughout the body. What’s more, the scientists found that ABG also competes to bind a wider variety of alkaloid toxins. 

Although Alvarez-Buylla’s work focused on frog toxin metabolism, it may have broader implications for bioengineering and poison therapy. “Studying weird animals is useful because you find examples of how nature has crafted clever solutions to problems, problems we may run into in the future.” 

The results of this research project will be presented by Aurora Alvarez-Buylla at SICB 2023 in Austin, TX.