Saxitoxin Is a Gating Modifier of hERG K+ Channels

Abstract

Potassium (K+) channels mediate numerous electrical events in excitable cells, including cellular membrane potential repolarization. The hERG K+ channel plays an important role in myocardial repolarization, and inhibition of these K+ channels is associated with long QT syndromes that can cause fatal cardiac arrhythmias. In this study, we identify saxitoxin (STX) as a hERG channel modifier and investigate the mechanism using heterologous expression of the recombinant channel in HEK293 cells. In the presence of STX, channels opened slower during strong depolarizations, and they closed much faster upon repolarization, suggesting that toxin-bound channels can still open but are modified, and that STX does not simply block the ion conduction pore. STX decreased hERG K+ currents by stabilizing closed channel states visualized as shifts in the voltage dependence of channel opening to more depolarized membrane potentials. The concentration dependence for steady-state modification as well as the kinetics of onset and recovery indicate that multiple STX molecules bind to the channel. Rapid application of STX revealed an apparent “agonist-like” effect in which K+ currents were transiently increased. The mechanism of this effect was found to be an effect on the channel voltage-inactivation relationship. Because the kinetics of inactivation are rapid relative to activation for this channel, the increase in K+ current appeared quickly and could be subverted by a decrease in K+ currents due to the shift in the voltage-activation relationship at some membrane potentials. The results are consistent with a simple model in which STX binds to the hERG K+ channel at multiple sites and alters the energetics of channel gating by shifting both the voltage-inactivation and voltage-activation processes. The results suggest a novel extracellular mechanism for pharmacological manipulation of this channel through allosteric coupling to channel gating.

Emerging health issues of cyanobacterial blooms

“…Anatoxin-a(s) irreversibly inhibits acetylcholinesterase (AChE) in the neuro-muscular junctions in the peripheral nervous system only. Saxitoxins (STXs) are a family of more than 30 natural alkaloids, synthesized by both freshwater cyanobacteria and by marine dinoflagellates by similar processes [51]: they block Na-channels in neuronal cells [52] and Ca ++ and K + channels in cardiac cells, thus preventing the propagation of electrical transmission within the peripheral nerves and skeletal or cardiac muscles [53]. Based on the kind of intoxication, they are also called Paralytic Shellfish Poisoning (PSP).…”

First report of cyanobacterial paralytic shellfish toxin biosynthesis genes and paralytic shellfish toxin production in Polish freshwater lakes

“…PSTs affect vertebrate nerve function by reversibly blocking voltage-gated ion channels, primarily Na + (Llewellyn, 2006), but also K + and Ca 2+ channels are affected (Kao, 1986, Wang et al, 2003, Su et al, 2004. Ion channel blockage leads to inhibition of neuronal signal transduction, the extent of which is directly dependent on PST concentration.…”

Localization of the ergtoxin-1 receptors on the voltage sensing domain of hERG K+ channel by AFM recognition imaging

“…There are two major classes of toxins, the pore-blocking [29,47] and the gatingmodifying [48,49], which have been widely used to determine K + channel structure and function. The interaction between toxins and the channel was studied by the two-electrode voltage-clamp recording techniques by many researchers.…”

“…The last is sensitive to changes in ion concentration [51] and to tetraethyl ammonium (known as external pore blocker) as well [52]. On the other hand, gating-modifier toxins bind to the voltage-sensor domain [48,49]. It has been found that ErgTx1 binds to the outer vestibule of hERG [26,27] like other pore blocking toxins and sensitive to tetraethyl ammonium [28].…”