Introduction 

Huwentoxin IV, a 35-amino-acid-residue polypeptide from Chinese tarantula Ornithoctonus huwena venom, is a kind of selective Na_V1.7 channel blocker. It preferentially inhibits neuronal Na_V1.7, 1.2 and 1.3 (IC50 values are 26, 150 and 338 nM respectively), compared to muscle subtypes NaV1.4 and 1.5 (IC50 = >10 μM).

Pharmacologic action

Huwentoxin IV was a sodium channel antagonist preferentially targeting peripheral isoforms via a mechanism quite different from ProTx-II. Huwentoxin IV lacked the ability of partitioning into phospholipid bilayer of artificial membranes, compared with ProTx-II. Huwentoxin IV exhibits high sequence similarity to other tarantula toxins, such as ProTx-II, hainantoxin-I, CcoTx2 and hainantoxin-IV (57%, 66%, 77% and 80%, respectively). Their divergent residues determine their responses to different sodium channel subtypes with the distinct biological potencies. The structure of huwentoxin IV represents a typical inhibitor cystine knot motif frequently emerging in spiders and cone snails. Na_V1.7 is a voltage-gated sodium channel involved in the generation and conduction of neuropathic and nociceptive pain signals. Huwentoxin IV inhibits the channel by binding at the neurotoxin receptor site 4 in the S3-S4 linker of domain II and trapping the voltage sensor in the inward and closed configuration.

Function

The sodium channel isoforms from rat hippocampus neurons were sensitive to huwentoxin-IV, but the toxin-binding affinity (IC50 0.4 μM) was 12-time lower than peripheral isoforms. And because of the dissociation of toxin–channel complex as observed for receptor site 3 toxins, the blockade by huwentoxin-IV could be reversed by strong depolarization. Moreover, the small unilamellar vesicle-binding assays suggested that huwentoxin-IV almost lacked the ability of partitioning into the negatively charged and neutral phospholipid bilayer of artificial membranes, in contrast to ProTx-II from the tarantula Thrixopelma pruriens. In accordance with the observation on rat hippocampus neurons, huwentoxin IV can completely depress all induced TTX-sensitive sodium currents on DRG neurons, suggesting that all these subtypes are sensitive to the toxin.

Pharmacokinetics and metabolism

When the concentration of toxin was increased to 1 μM. The blockade by 0.5 or 1 μM huwentoxin IV did not change the shape of inward current traces, indicating that the peptides did not alter channel activation and inactivation kinetics. Besides, huwentoxin IV at 1 μM had no effect on TTX-resistant sodium channels on adult rat DRG neurons.

References:

1. Xiao, Y., Luo, X., Kuang, F., Deng, M., Wang, M., Zeng, X., & Liang, S. (2008). Synthesis and characterization of huwentoxin-IV, a neurotoxin inhibiting central neuronal sodium channels. Toxicon, 51(2), 230-239.

2. Revell, J. D., Lund, P. E., Linley, J. E., Metcalfe, J., Burmeister, N., Sridharan, S & Bednarek, M. A. (2013). Potency optimization of Huwentoxin-IV on hNav1. 7: a neurotoxin TTX-S sodium-channel antagonist from the venom of the Chinese bird-eating spider Selenocosmia huwena. Peptides, 44, 40-46.