Hainantoxin

'''Hainantoxins''' ('''HNTX''') are neurotoxins from the venom of the Chinese bird spider ''Cyriopagopus hainanus''. Hainantoxins specifically inhibit tetrodotoxin-sensitive Voltage-gated sodium channels, thereby causing blockage of neuromuscular transmission and paralysis.<ref name="lietal1" /><ref name="xiao2" /> Currently, 13 different hainantoxins are known (HNTX-I – HNTX-XIII), but only HNTX-I, -II, -III, -IV and -V have been investigated in detail.<ref name="site3" />

==Sources== HNTX-I, HNTX-III, HNTX-IV and HNTX-V are made by the Chinese bird spider ''Haplopelma hainanum'' (=''Ornithoctonus hainana'', ''Selenocosmia hainana'').<ref name="lietal1" /><ref name="xiao2" /><ref name="function4" /><ref name="xuxetal5" /><ref name="zeng6" /><ref name="honma7" /><ref name="liang8" /><ref name="spyc9" /><ref name="liuyetal10" /><ref name="xiong11" />

==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.<ref name="luiz12" /> 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.<ref name="function4" /> The amino acid residues His28 and Asp26 are needed for the bioactivity of HNTX-I.<ref name="nichol13" />

==== 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.<ref name="pan15" />

==== 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.<ref name="zeng6" /><ref name="liang8" />

==== 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.<ref name="xiong11" /> The complete amino acid sequence of HNTX-IV is NH2-ECLGFG KGCNPS NDQCCK SSNLVC SRKHRW CKYEI-CONH2.<ref name="xiong11" /> Lys 27, His28, Arg29 and Lys 32 are the neuroactive amino acid residues.<ref name="lietal1" /><ref name="xuxetal5" /><ref name="liuyetal10" />

==== HNTX-V ==== HNTX-V consists of 35 amino acid residues.<ref name="xiao2" /> 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.<ref name="xiao2" />

==Target==

=== Channel ===

Hainantoxins selectively inhibit tetrodotoxin-sensitive (TTX-S) voltage-gated sodium channels (VGSCs).<ref name="lietal1" /><ref name="xuxetal5" /><ref name="zeng6" /><ref name="spyc9" /> Voltage-gated Ca2+ channels (VGCCs), tetrodotoxin-resistant (TTX-R) VGSCs and rectifier-delayed potassium channels are not affected.<ref name="liang8" /> HNTX-III and HNTX-IV are part of the Huwentoxin-I family.<ref name="site3" /><ref name="liang8" /> 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.<ref name="function4" /><ref name="liuyetal10" />

=== 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.<ref name="lietal1" /><ref name="xiao2" /> In HNTX-I, the positively charged residues and a vicinal hydrophobic patch have most influence on the binding to the sodium channels.<ref name="function4" /> 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.<ref name="liuyetal10" /><ref name="wang14" /> 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.<ref name="function4" />

Table 1: IC50 values of four subgroups of hainantoxins {| class="IC50 values of four hainantoxins" |- ! !! IC50 |- | '''HNTX-I''' || 68 μM<ref name="function4" /> |- | '''HNTX-III''' || 1.1 nM<ref name="liang8" /> |- | '''HNTX-IV''' || 44.6 nM<ref name="liang8" /> |- | '''HNTX-V''' || 42.3 nM<ref name="xiao2" /> |}

==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.<ref name="lietal1" /><ref name="function4" /> Ion selectivity of the VGSCs is not changed by hainantoxin.<ref name="liang8" /><ref name="spyc9" /> The mode of action of HNTX-II is unclear, but is unlikely to involve sodium channels.<ref name="pan15" />

==Toxicity==

=== Symptoms ===

Hainantoxins can affect both vertebrates and invertebrates. HNTX-I has no significant effect on insects or rats.<ref name="xiao2" /><ref name="luiz12" /> HNTX-III and HNTX-IV cause spontaneous contractions of the diaphragm muscle and the vas deferens smooth muscle of the rat.<ref name="liang8" /><ref name="spyc9" /> HNTX-III and HNTX-IV are able to paralyze cockroaches, and HNTX-IV can even paralyze rats.<ref name="wang14" />

=== LD₅₀ === Intracerebroventricular injection in mice with HNTX-II shows an {{LD50}} of 1.41 μg/g. The intraperitoneal LD₅₀ value of HNTX-IV in mice is 0.2&nbsp;mg/kg.<ref name="liang8" /><ref name="spyc9" /> HNTX-III is 40 times more potent than HNTX-IV.<ref name="liang8" />

==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''.<ref name="liang8" />

== References == {{Reflist| refs= <ref name="lietal1">Li D et al. Structure--activity relationships of hainantoxin-IV and structure determination of active and inactive sodium channel blockers. J Biol Chem. 2004 Sep 3;279(36):37734-40. Epub 2004 Jun 16.</ref> <ref name="xiao2">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.</ref> <ref name="site3">{{Cite web|url=https://www.uniprot.org/uniprot/?query=family:%22huwentoxin-1+family%22|title = Family:%22huwentoxin-1 family%22 in UniProtKB}}</ref> <ref name="function4">Li D, et al. Function and solution structure of hainantoxin-I, a novel insect sodium channel inhibitor from the Chinese bird spider Selenocosmia hainana. FEBS Lett. 2003 Dec 18;555(3):616-22.</ref> <ref name="xuxetal5">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.</ref> <ref name="zeng6">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.</ref> <ref name="honma7">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.</ref> <ref name="liang8">Xiao Y, Liang S. Inhibition of neuronal tetrodotoxin-sensitive Na+ channels by two spider toxins: hainantoxin-III and hainantoxin-IV. Eur J Pharmacol. 2003 Sep 5;477(1):1-7.</ref> <ref name="spyc9">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.</ref> <ref name="liuyetal10">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.</ref> <ref name="xiong11">XIONG Xia et al. Effects of Arg26 and Lys27 mutation on the bioactivity of HNTX-IV</ref> <ref name="luiz12">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.</ref> <ref name="nichol13">Nicholson GM. Insect-selective spider toxins targeting voltage-gated sodium channels. Toxicon. 2007 Mar 15;49(4):490-512. Epub 2006 Dec 5.</ref> <ref name="wang14">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.</ref> <ref name="pan15">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.</ref> }}

Category:Neurotoxins Category:Ion channel toxins Category:Spider toxins