Hainantoxin

Hainantoxins (HNTX) are neurotoxins from the venom of the Chinese bird spider '' Haplopelma hainanum''. Hainantoxins specifically inhibit tetrodotoxin-sensitive Voltage-gated sodium channels, thereby causing blockage of neuromuscular transmission and paralysis. Currently, 13 different hainantoxins are known (HNTX-I – HNTX-XIII), but only HNTX-I, -II, -III, -IV and -V have been investigated in detail.

Sources

HNTX-I, HNTX-III, HNTX-IV and HNTX-V are made by the Chinese bird spider ''Haplopelma hainanum'' (=''Ornithoctonus hainana'', ''Selenocosmia hainana'').

Chemistry

Structure

Hainantoxins I, III, IV and V show high homology, including the presence of three disulfide bonds that form an inhibitor cysteine knot (ICK) motif.

HNTX-I

The main component of the venom of ''O. hainana'' is HNTX-I. It has 33 amino acid residues, with a total molecular weight of 3605-3608 Da. HNTX-I contains a short triple-stranded anti-parallel beta-sheet and four beta-turns. The amino acid residues His28 and Asp26 are needed for the bioactivity of HNTX-I.

HNTX-II

HNTX-II has a molecular weight of 4253 Da and contains 37 amino acid residues. The complete amino acid sequence of HNTX-II is NH2-LFECSV SCEIEK EGNKD CKKKK CKGGW KCKFN MCVKV-COOH.

HNTX-III

The structure of HNTX-III consists of 33-35 amino acid residues, which form a beta-sheet with connections between Asp7 and Cys9, Tyr21 and Ser23, and Lys27 and Val30.

HNTX-IV

HNTX-IV has 35 amino acid residues with a total molecular weight of 3989 Da. The first strand consists of an antiparallel beta-sheet. The complete amino acid sequence of HNTX-IV is NH2-ECLGFG KGCNPS NDQCCK SSNLVC SRKHRW CKYEI-CONH2. Lys 27, His28, Arg29 and Lys 32 are the neuroactive amino acid residues.

HNTX-V

HNTX-V consists of 35 amino acid residues. The whole amino acid residue sequence of HNTX-V is NH2-ECLGFG KGCNPS NDQCCK SANLVC SRKHRW CKYEI-COOH. At the active binding site of HNTX-V, Lys27 and Arg 29 are the most important.

Target

Channel

Hainantoxins selectively inhibit tetrodotoxin-sensitive (TTX-S) voltage-gated sodium channels (VGSCs). Voltage-gated Ca2+ channels (VGCCs), tetrodotoxin-resistant (TTX-R) VGSCs and rectifier-delayed potassium channels are not affected. HNTX-III and HNTX-IV are part of the Huwentoxin-I family. Toxins from the Huwentoxin-I family are thought to bind to site 1 on the sodium channels. Other hainantoxins bind at site 3 of the sodium channels. HNTX-I specifically blocks mammalian Nav1.2 and insect para/tipE channels expressed in Xenopus laevis oocytes. HNTX-I is a weak antagonist of the vertebrate TTX-S VGSCs, but is more potent on insect VGSCs.

Affinity

For the blockage of sodium channels, electrostatic interactions or hydrogen bonds are needed. Important for the electrostatic interaction is the presence of a positively charged region in the toxin, because the receptor site of the sodium channel contains a lot of negatively charged residues. In HNTX-I, the positively charged residues and a vicinal hydrophobic patch have most influence on the binding to the sodium channels. HNTX-IV has a positively charged patch containing the amino acids Arg26, Lys27, His28, Arg29 and Lys32, of which Lys27, Arg29 and Lys32 are the most important for interaction with the TTX-S VGSCs. HNTX-V also shows an interface of positively charged amino acids that are responsible for the binding with the TTX-S VGSCs, where also Lys27 and Arg29 are the most important. Subtle differences in the positively charged patch can result in altered electrostatic properties, causing altered pharmacological effects.

Table 1: IC50 values of four subgroups of hainantoxins

Mode of action

HNTX-I, HNTX-III, HNTX-IV, and HNTX-V are thought to bind to site 1 of voltage-dependent sodium channels, similar to TTX, and thereby block the channel pore. They do not alter activation and inactivation kinetics. Ion selectivity of the VGSCs is not changed by hainantoxin. The mode of action of HNTX-II is unclear, but is unlikely to involve sodium channels.

Toxicity

Symptoms

Hainantoxins can affect both vertebrates and invertebrates. HNTX-I has no significant effect on insects or rats. HNTX-III and HNTX-IV cause spontaneous contractions of the diaphragm muscle and the vas deferens smooth muscle of the rat. HNTX-III and HNTX-IV are able to paralyze cockroaches, and HNTX-IV can even paralyze rats.

LD50

Intracerebroventricular injection in mice with HNTX-II shows a LD50 of 1.41 μg/g. The intraperitoneal LD50 value of HNTX-IV in mice is 0.2 mg/kg. HNTX-III is 40 times more potent that HNTX-IV.

Therapeutic use

HNTX-III and HNTX-IV have an antagonistic effect on the toxin BMK-I, a toxic protein in the venom of the scorpion Buthus martensii.

References

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Xiao YC, Liang SP. Purification and characterization of Hainantoxin-V, a tetrodotoxin-sensitive sodium channel inhibitor from the venom of the spider Selenocosmia hainana. Toxicon. 2003 May;41(6):643-50.
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Xu X et al. Solid-phase synthesis and biological characterization of S12A-HNTX-IV and R29A-HNTX-IV: two mutants of hainantoxin-IV. Sheng Wu Gong Cheng Xue Bao. 2005 Jan;21(1):92-6.
Zeng XZ et al. Sequence-specific assignment of 1H-NMR resonance and determination of the secondary structure of Jingzhaotoxin-I. Acta Biochim Biophys Sin (Shanghai). 2005 Aug;37(8):567-72.
Honma T et al. Novel peptide toxins from acrorhagi, aggressive organs of the sea anemone Actinia equina. Toxicon. 2005 Dec 1;46(7):768-74. Epub 2005 Sep 23.
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Xiao YC, Liang SP. Inhibition of sodium channels in rat dorsal root ganglion neurons by Hainantoxin-IV, a novel spider toxin. Sheng Wu Hua Xue Yu Sheng Wu Wu Li Xue Bao (Shanghai). 2003 Jan;35(1):82-5.
Liu Y et al. A positively charged surface patch is important for hainantoxin-IV binding to voltage-gated sodium channels. J Pept Sci. 2012 Oct;18(10):643-9. doi: 10.1002/psc.2451. Epub 2012 Aug 27.
XIONG Xia et al. Effects of Arg26 and Lys27 mutation on the bioactivity of HNTX-IV
Liu Z et al. Isolation and characterization of hainantoxin-IV, a novel antagonist of tetrodotoxin-sensitive sodium channels from the Chinese bird spider Selenocosmia hainana. Cell Mol Life Sci. 2003 May;60(5):972-8.
Nicholson GM. Insect-selective spider toxins targeting voltage-gated sodium channels. Toxicon. 2007 Mar 15;49(4):490-512. Epub 2006 Dec 5.
Wang RL et al. Mechanism of action of two insect toxins huwentoxin-III and hainantoxin-VI on voltage-gated sodium channels. J Zhejiang Univ Sci B. 2010 Jun;11(6):451-7.
Pan J-Y, Yu Z-Q. Isolation and characterization of Hainantoxin-II, a new neurotoxic peptide from the Chinese bird spider (''Haplopelma hainanum''). Zool. Res. 2010 6:570-4.

Neurotoxins
Ion channel toxins
Spider toxins