Eukaryotic voltage-gated sodium (Na v ) channels contribute to the rising phase of action potentials and served as an early muse for biophysicists laying the foundation for our current understanding of electrical signaling. Given their central role in electrical excitability, it is not surprising that (a) inherited mutations in genes encoding for Na v channels and their accessory subunits have been linked to excitability disorders in brain, muscle, and heart; and (b) Na v channels are targeted by various drugs and naturally occurring toxins. Although the overall architecture and behavior of these channels are likely to be similar to the more well-studied voltage-gated potassium channels, eukaryotic Na v channels lack structural and functional symmetry, a notable difference that has implications for gating and selectivity. Activation of voltage-sensing modules of the first three domains in Na v channels is sufficient to open the channel pore, whereas movement of the domain IV voltage sensor is correlated with inactivation. Also, structure–function studies of eukaryotic Na v channels show that a set of amino acids in the selectivity filter, referred to as DEKA locus, is essential for Na + selectivity. Structures of prokaryotic Na v channels have also shed new light on mechanisms of drug block. These structures exhibit lateral fenestrations that are large enough to allow drugs or lipophilic molecules to gain access into the inner vestibule, suggesting that this might be the passage for drug entry into a closed channel. In this Review, we will synthesize our current understanding of Na v channel gating mechanisms, ion selectivity and permeation, and modulation by therapeutics and toxins in light of the new structures of the prokaryotic Na v channels that, for the time being, serve as structural models of their eukaryotic counterparts.

Animal toxins that inhibit voltage-gated sodium (Na v ) channel fast inactivation can do so through an interaction with the S3b–S4 helix-turn-helix region, or paddle motif, located in the domain IV voltage sensor. Here, we used surface plasmon resonance (SPR), an optical approach that uses polarized light to measure the refractive index near a sensor surface to which a molecule of interest is attached, to analyze interactions between the isolated domain IV paddle and Na v channel–selective α-scorpion toxins. Our SPR analyses showed that the domain IV paddle can be removed from the Na v channel and immobilized on sensor chips, and suggest that the isolated motif remains susceptible to animal toxins that target the domain IV voltage sensor. As such, our results uncover the inherent pharmacological sensitivities of the isolated domain IV paddle motif, which may be exploited to develop a label-free SPR approach for discovering ligands that target this region.

The voltage-activated sodium (Nav) channel Nav1.9 is expressed in dorsal root ganglion (DRG) neurons where it is believed to play an important role in nociception. Progress in revealing the functional properties and pharmacological sensitivities of this non-canonical Nav channel has been slow because attempts to express this channel in a heterologous expression system have been unsuccessful. Here, we use a protein engineering approach to dissect the contributions of the four Nav1.9 voltage sensors to channel function and pharmacology. We define individual S3b–S4 paddle motifs within each voltage sensor, and show that they can sense changes in membrane voltage and drive voltage sensor activation when transplanted into voltage-activated potassium channels. We also find that the paddle motifs in Nav1.9 are targeted by animal toxins, and that these toxins alter Nav1.9-mediated currents in DRG neurons. Our results demonstrate that slowly activating and inactivating Nav1.9 channels have functional and pharmacological properties in common with canonical Nav channels, but also show distinctive pharmacological sensitivities that can potentially be exploited for developing novel treatments for pain.