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

Abstract

Many poisonous organisms carry small-molecule toxins that alter voltage-gated sodium channel (NaV) function. Among these, batrachotoxin (BTX) from Pitohui poison birds and Phyllobates poison frogs stands out because of its lethality and unusual effects on NaV function. How these toxin-bearing organisms avoid autointoxication remains poorly understood. In poison frogs, a NaV 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 NaVs, incurs a strong cost compromising channel function, and fails to produce BTX-resistant channels in poison frog NaVs. We also show that captivity-raised poison frogs are resistant to two NaV-directed toxins, BTX and saxitoxin (STX), even though they bear NaVs sensitive to both. Moreover, we demonstrate that the amphibian STX “toxin sponge” protein saxiphilin is able to protect and rescue NaVs 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.

Are Pacific Chorus Frogs (Pseudacris regilla) Resistant to Tetrodotoxin (TTX)? Characterizing Potential TTX Exposure and Resistance in an Ecological Associate of Pacific Newts (Taricha)

“…4). However, amino acid changes in important regions of the Nav1.4 protein do not always translate to a resistant phenotype (e.g., see Abderemane-Ali et al, 2021). Furthermore, both allopatric and sympatric P. regilla possessed the K.O.…”

“…Montana et al Tetrodotoxin Resistance in Pseudacris regilla 6 barriers, which are present in some invertebrates (Mebs et al, 2016). Some amphibians possess the toxin-binding, blood-plasma protein saxiphilin, but saxiphilin is incapable of binding TTX, at least in the animals that have been tested (Mahar et al, 1991;Llewellyn et al, 1997;Abderemane-Ali et al, 2021). Some species of mantids (Tenodera sinensis, Sphodromantis viridis, Hierodula membranacea, and Miomantis caffra) can survive tetrodotoxin ingestion but not toxin injection, suggesting the presence of a gut diffusion barrier (Mebs et al, 2016).…”

How do batrachotoxin-bearing frogs and birds avoid self intoxication?

“…Such instances of replicated evolution provide excellent opportunities to draw generalities about the evolutionary processes and functional mechanisms underlying toxicity and toxin resistance, as well as the biochemical and physiological properties of toxins themselves. In an earlier issue of the Journal of General Physiology , Abderemane-Ali et al (2021) address the physiological mechanisms through which frogs and birds that secrete the deadly neurotoxin batrachotoxin (BTX) are able to resist its noxious effects. Their results challenge previous ideas on BTX resistance and provide exciting new hypotheses and future directions.…”

“… Abderemane-Ali et al (2021) conducted BTX resistance assays on voltage-gated sodium channels of Pitohui uropygialis and Ph. terribilis.…”

“…However, in retrospect, this scenario may not be the most likely from an evolutionary perspective: although mutations that confer target-site insensitivity are a commonly invoked mechanism for neurotoxin resistance (e.g., Geffeney et al, 2005 ; Zhen et al, 2012 ; Martinez-Torres et al, 1999 ), they are often accompanied by functional costs since they involve meddling with functionally important regions of highly fine-tuned proteins ( Hague et al, 2018 )⁠. This was made evident by the functional costs observed in BTX-resistant bird and mammalian channels generated by Abderemane-Ali et al (2021) . Since BTX is able to modify many of the different voltage-gated sodium channels encoded in vertebrate genomes (e.g., Bosmans et al, 2004 ; Wang and Wang, 1998 ; Li et al, 2002 ; Linford et al, 1998 ; Wang et al, 2007 ), self-resistance through target-site insensitivity at multiple channels could potentially result in major functional tradeoffs at the organismal level.…”