Non-blocking modulation contributes to sodium channel inhibition by a covalently attached photoreactive riluzole analog

Peter Lukacs,Emilio Casanova,Luca Valánszki,András Málnási-Csizmadia,Arpad Mike,László Nyitray,Beáta Biri-Kovács,Mátyás C. Földi

doi:10.1038/s41598-018-26444-y

Abstract

Sodium channel inhibitor drugs decrease pathological hyperactivity in various diseases including pain syndromes, myotonia, arrhythmias, nerve injuries and epilepsies. Inhibiting pathological but not physiological activity, however, is a major challenge in drug development. Sodium channel inhibitors exert their effects by a dual action: they obstruct ion flow (“block”), and they alter the energetics of channel opening and closing (“modulation”). Ideal drugs would be modulators without blocking effect, because modulation is inherently activity-dependent, therefore selective for pathological hyperactivity. Can block and modulation be separated? It has been difficult to tell, because the effect of modulation is obscured by conformation-dependent association/dissociation of the drug. To eliminate dynamic association/dissociation, we used a photoreactive riluzole analog which could be covalently bound to the channel; and found, unexpectedly, that drug-bound channels could still conduct ions, although with modulated gating. The finding that non-blocking modulation is possible, may open a novel avenue for drug development because non-blocking modulators could be more specific in treating hyperactivity-linked diseases.

Highlights

Voltage-gated sodium channels (VGSC) are essential components of electrical signal propagation in excitable tissues
Common sodium channel inhibitor drugs are state-dependent: they produce a weaker inhibition at hyperpolarized membrane potentials, which is assumed to be due to channel block, and a much stronger inhibition at depolarized membrane potentials, which is thought to be due to a higher degree of channel block and, in addition, to modulation as well
Riluzole concentrations are higher than what was found effective in decreasing firing rate, we sought to understand the mechanism by using concentrations that could ensure fast association and high occupancy of binding sites

Summary

Introduction

Voltage-gated sodium channels (VGSC) are essential components of electrical signal propagation in excitable tissues. Most sodium channel inhibitors exert a certain degree of functional selectivity, showing a definite preference for cells with abnormally high activity or a slightly depolarized membrane potential. Is typically inactivated conformation, a state assumed by the channel upon prolonged depolarization (either after opening or even without opening), which is essential in preventing overexcitation, and in making signal propagation by self-regenerating sodium channel activation. Common sodium channel inhibitor drugs are state-dependent: they produce a weaker inhibition at hyperpolarized membrane potentials, which is assumed to be due to channel block, and a much stronger inhibition at depolarized membrane potentials, which is thought to be due to a higher degree of channel block and, in addition, to modulation as well. An ideal sodium channel inhibitor, should prevent hyperexcitability, while not affecting normal excitability For this reason, state-dependent inhibition is more desirable than resting channel block. In the case of riluzole, a drug with an exceptionally high state-dependence (see below), we will investigate both possibilities

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