Evidence that toxin resistance in poison birds and frogs is not rooted in sodium channel mutations and may rely on “toxin sponge” proteins

Many poisonous organisms carry small-molecule toxins that alter voltage-gated sodium channel (Na(V)) function. Among these, batrachotoxin (BTX) from Pitohui poison birds and Phyllobates poison frogs stands out because of its lethality and unusual effects on Na(V) function. How these toxin-bearing organisms avoid autointoxication remains poorly understood. In poison frogs, a Na(V) DIVS6 pore-forming helix N-to-T mutation has been proposed as the BTX resistance mechanism. Here, we show that this variant is absent from Pitohui and poison frog Na(V)s, incurs a strong cost compromising channel function, and fails to produce BTX-resistant channels in poison frog Na(V)s. We also show that captivity-raised poison frogs are resistant to two Na(V)-directed toxins, BTX and saxitoxin (STX), even though they bear Na(V)s sensitive to both. Moreover, we demonstrate that the amphibian STX “toxin sponge” protein saxiphilin is able to protect and rescue Na(V)s from block by STX. Taken together, our data contradict the hypothesis that BTX autoresistance is rooted in the DIVS6 N→T mutation, challenge the idea that ion channel mutations are a primary driver of toxin resistance, and suggest the possibility that toxin sequestration mechanisms may be key for protecting poisonous species from the action of small-molecule toxins.