Huwentoxin IV, a 35-amino-acid-residue polypeptide from Chinese tarantula Ornithoctonus huwena venom, is a kind of selective NaV1.7 channel blocker. It preferentially inhibits neuronal NaV1.7, 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).
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. NaV1.7 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.
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.
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