ProTx-II, a Selective Inhibitor of NaV1.7 Sodium Channels, Blocks Action Potential Propagation in Nociceptors

Voltage-gated sodium (NaV1) channels play a critical role in modulating the excitability of sensory neurons, and human genetic evidence points to NaV1.7 as an essential contributor to pain signaling. Human loss-of-function mutations in SCN9A, the gene encoding NaV1.7, cause channelopathy-associated indifference to pain (CIP), whereas gain-of-function mutations are associated with two inherited painful neuropathies. Although the human genetic data make NaV1.7 an attractive target for the development of analgesics, pharmacological proof-of-concept in experimental pain models requires NaV1.7-selective channel blockers. Here, we show that the tarantula venom peptide ProTx-II selectively interacts with NaV1.7 channels, inhibiting NaV1.7 with an IC50 value of 0.3 nM, compared with IC50 values of 30 to 150 nM for other heterologously expressed NaV1 subtypes. This subtype selectivity was abolished by a point mutation in DIIS3. It is interesting that application of ProTx-II to desheathed cutaneous nerves completely blocked the C-fiber compound action potential at concentrations that had little effect on Aβ-fiber conduction. ProTx-II application had little effect on action potential propagation of the intact nerve, which may explain why ProTx-II was not efficacious in rodent models of acute and inflammatory pain. Mono-iodo-ProTx-II (125I-ProTx-II) binds with high affinity (Kd = 0.3 nM) to recombinant hNaV1.7 channels. Binding of 125I-ProTx-II is insensitive to the presence of other well characterized NaV1 channel modulators, suggesting that ProTx-II binds to a novel site, which may be more conducive to conferring subtype selectivity than the site occupied by traditional local anesthetics and anticonvulsants. Thus, the 125I-ProTx-II binding assay, described here, offers a new tool in the search for novel NaV1.7-selective blockers.