The major component of Heteractis magnifica sea anemone venom, RpIII, exhibits strong subtype selectivity for insects over mammalian voltage-gated sodium channels

Abstract

Voltage-gated sodium channels (Na_V) are molecular targets for the development of drugs for the treatment of diseases such as epilepsy, neuropathic pain, long QT syndrome, etc., as well as for insecticides. Therefore, the search for novel selective Na_V channel ligands is relevant. Using amplicon deep sequencing of tentacle cDNA libraries from sea anemones Heteractis magnifica, 36 transcripts related to RpIII neurotoxin, a Na_V channel modulators, were revealed. The recombinant RpIII was moderately toxic for mice (LD₅₀ 0.030 ± 0.004 mg/kg) but did not demonstrate any activity towards Na_V in human SH-SY5Y cells. The toxin inhibited inactivation of heterologously expressed mammalian, insect, and arachnid Na_V channels with higher specificity to insect channels. Cockroach (Blattella germanica) sodium channel BgNa_V1 (EC₅₀ of 2.4 ± 0.2 nM) and yellow fever mosquito (Aedes aegypti) channel AaNa_V1 (EC₅₀ of 1.5 ± 0.3 nM) were the most sensitive to RpIII, while mammals Na_V had EC₅₀ values above 100 nM except mNa_V1.6 (EC₅₀ of 43.8 ± 3.6 nM). The low nanomolar RpIII affinity to insect AaNa_V1 may be explained by the extensive intermolecular contacts found by docking study. According to the predicted data, the toxin lands on the ion channel between voltage-sensing domain IV and pore domain I, also known as toxin site 3, followed by stabilizing the channels in the open state what was measured at electrophysiological experiments.