Functional modulation of the human voltage-gated sodium channel NaV1.8 by auxiliary β subunits

Biophysical mechanisms underlying tefluthrin-induced modulation of gating changes and resurgent current generation in the human Nav1.4 channel

Human skeletal muscle contraction is triggered by activation of Nav1.4 channels. Nav1.4 channels can generate resurgent currents by channel reopening at hyperpolarized potentials through a gating transition dependent on the intracellular Navβ4 peptide in the physiological conditions. Tefluthrin (TEF) is a pyrethroid insecticide that can disrupt electrical signaling in nerves and skeletal muscle, resulting in seizures, muscle spasms, fasciculations, and mental confusion. TEF can also induce tail currents through other voltage-gated sodium channels in the absence of Navβ4 peptide, suggesting that muscle spasms may be caused by resurgent currents. Further, intracellular Navβ4 peptide and extracellular TEF may show competitive or synergistic effects; however, their binding sites are still unknown. To address these issues, electrophysiological recordings were performed on CHO-K1 cells expressing Nav1.4 channels with intracellular Navβ4 peptide, extracellular TEF, or both. TEF and Navβ4 peptide induced a hyperpolarizing shift of activation and inactivation curves in the Nav1.4 channel. TEF also substantially prolonged the inactivation time constants, while simultaneous application of Navβ4 peptide partially reversed this effect. Resurgent currents were enhanced by TEF and Navβ4 peptide at negative potentials, but TEF more potently enhances resurgent currents and dampens decay of resurgent currents. With longer depolarization, peak resurgent currents decay was fastest with the TEF alone. Molecular docking suggested that TEF and Navβ4 peptide binding site(s) are not in the narrowest part of the channel pore, but rather in the bundle-crossing regions and in the domain linkers, respectively. TEF can induce resurgent currents independently and synergistically with Navβ4 peptide, which may explain the muscle spasms observed in TEF intoxication.

Advancements in the use of xenopus oocytes for modelling neurological disease for novel drug discovery

INTRODUCTION

Introduced about 50 years ago, the model of Xenopus oocytes for the expression of recombinant proteins has gained a broad spectrum of applications. The authors herein review the benefits brought from using this model system, with a focus on modeling neurological disease mechanisms and application to drug discovery.

AREAS COVERED

Using multiple examples spanning from ligand gated ion channels to transporters, this review presents, in the light of the latest publications, the benefits offered from using Xenopus oocytes. Studies range from the characterization of gene mutations to the discovery of novel treatments for disorders of the central nervous system (CNS).

EXPERT OPINION

Development of new drugs targeting CNS disorders has been marked by failures in the translation from preclinical to clinical studies. As progress in genetics and molecular biology highlights large functional differences arising from a single to a few amino acid exchanges, the need for drug screening and functional testing against human proteins is increasing. The use of Xenopus oocytes to enable precise modeling and characterization of clinically relevant genetic variants constitutes a powerful model system that can be used to inform various aspects of CNS drug discovery and development.

The β3-subunit modulates the effect of venom peptides ProTx-II and OD1 on NaV 1.7 gating

The voltage-gated sodium channel NaV 1.7 is involved in various pain phenotypes and is physiologically regulated by the NaV -β3-subunit. Venom toxins ProTx-II and OD1 modulate NaV 1.7 channel function and may be useful as therapeutic agents and/or research tools. Here, we use patch-clamp recordings to investigate how the β3-subunit can influence and modulate the toxin-mediated effects on NaV 1.7 function, and we propose a putative binding mode of OD1 on NaV 1.7 to rationalise its activating effects. The inhibitor ProTx-II slowed the rate of NaV 1.7 activation, whilst the activator OD1 reduced the rate of fast inactivation and accelerated recovery from inactivation. The β3-subunit partially abrogated these effects. OD1 induced a hyperpolarising shift in the V1/2 of steady-state activation, which was not observed in the presence of β3. Consequently, OD1-treated NaV 1.7 exhibited an enhanced window current compared with OD1-treated NaV 1.7-β3 complex. We identify candidate OD1 residues that are likely to prevent the upward movement of the DIV S4 helix and thus impede fast inactivation. The binding sites for each of the toxins and the predicted location of the β3-subunit on the NaV 1.7 channel are distinct. Therefore, we infer that the β3-subunit influences the interaction of toxins with NaV 1.7 via indirect allosteric mechanisms. The enhanced window current shown by OD1-treated NaV 1.7 compared with OD1-treated NaV 1.7-β3 is discussed in the context of differing cellular expressions of NaV 1.7 and the β3-subunit in dorsal root ganglion (DRG) neurons. We propose that β3, as the native binding partner for NaV 1.7 in DRG neurons, should be included during screening of molecules against NaV 1.7 in relevant analgesic discovery campaigns.

Late Sodium Current of the Heart: Where Do We Stand and Where Are We Going?

Late sodium current has long been linked to dysrhythmia and contractile malfunction in the heart. Despite the increasing body of accumulating information on the subject, our understanding of its role in normal or pathologic states is not complete. Even though the role of late sodium current in shaping action potential under physiologic circumstances is debated, it’s unquestioned role in arrhythmogenesis keeps it in the focus of research. Transgenic mouse models and isoform-specific pharmacological tools have proved useful in understanding the mechanism of late sodium current in health and disease. This review will outline the mechanism and function of cardiac late sodium current with special focus on the recent advances of the area.