Nixon, S. A. | bioRxiv Science

bioRxiv2025

Saxiphilin functions as a "toxin sponge" protein that counteracts the effects of saxitoxin poisoning

Saxitoxin (STX) is among the most potent toxins known, is classified as a chemical weapon, and is the archetype of the paralytic shellfish toxin (PST) family produced by marine and freshwater harmful algal blooms (HAB)1-3. STX causes paralysis and death through inhibition of voltage-gated sodium channels (NaVs), has no antidote, and poses a public health and commercial fishing threat due to its accumulation in seafood and increasing HAB occurrences1,4. Although STX is lethal to varied vertebrates5-7, including humans4,8,9, certain frogs resist STX poisoning6,7. This phenotype is thought to depend on soluble toxin sponge high-affinity STX binding proteins (saxiphilins, Sxphs)6,7,10,11 that provide resistance through a competition mechanism different from classic target site ion channel mutation mechanisms12-14. Here, we test this idea directly and show that a single dose of American bullfrog (Rana catesbeiana) Sxph (RcSxph) is sufficient to counteract STX neurotoxicity and lethality in mice using paradigms where STX and Sxph are administered together or sequentially in either order. Importantly, this function requires the RcSxph high-affinity STX binding site. Our findings provide in vivo validation that Sxphs are toxin sponges that protect against STX poisoning and highlight the potential to harness this protein class as antidotes for PSTs and other toxins.

pharmacology and toxicology↗

bioRxiv2024

Structural basis for saxitoxin congener binding and neutralization by anuran saxiphilins

Dinoflagellates and cyanobacteria in harmful red tide algal blooms produce saxitoxin (STX) and [~]50 congeners that block voltage-gated sodium channel (NaV) function and disrupt bioelectrical signals1-4. Consuming seafood carrying these lethal toxins causes paralytic shellfish poisoning (PSP), a growing public health hazard due to climate change5-7 that motivates efforts to detect these toxins and counteract their noxious effects. Although structural studies of NaVs8,9 and anuran soluble STX binding proteins known as saxiphilins (Sxphs)10,11 revealed convergent binding modes for the bis-guanidinium STX core10,11, the structural basis for STX congener recognition is unknown. Here, we show that American bullfrog (Rana catesbeiana) RcSxph10,11 and High Himalaya frog (Nanorana parkeri) NpSxph10 use a pre-organized pocket to sequester STX congeners through a binding mode shared with STX. This lock and key recognition yields a tradeoff between a relatively rigid high-affinity toxin binding site in which bound waters are crucial and the ability of Sxphs to accommodate STX congener modifications. Importantly, functional studies show that Sxphs act as toxin sponges that reverse NaV block by multiple STX congeners and can detect these bis-guanidinium toxins in a radioligand receptor binding assay (RBA) for PSP toxin environmental testing12,13. Our findings establish how Sxphs sequester diverse neurotoxins and reveal structural factors underlying STX congener binding differences between Sxphs and NaVs that are rooted in the distinct toxin binding orientations on these two targets. These insights expand the molecular foundation required for understanding toxin sponge action and for guiding development of new means to monitor PSTs and mitigate their harmful effects.

biophysics↗