Abstract
Activation of voltage-gated ion channels is regulated by conformational changes of the voltage sensor domains (VSDs), four water- and ion-impermeable modules peripheral to the central, permeable pore domain. Anomalous currents, defined as ω-currents, have been recorded in response to mutations of residues on the VSD S4 helix and associated with ion fluxes through the VSDs. In humans, gene defects in the potassium channel Kv7.2 result in a broad range of epileptic disorders, from benign neonatal seizures to severe epileptic encephalopathies. Experimental evidence suggests that the R207Q mutation in S4, associated with peripheral nerve hyperexcitability, induces ω-currents at depolarized potentials, but the fine structural details are still elusive. In this work, we use atom-detailed molecular dynamics simulations and a refined model structure of the Kv7.2 VSD in the active conformation in a membrane/water environment to study the effect of R207Q and four additional mutations of proven clinical importance. Our results demonstrate that the R207Q mutant shows the most pronounced increase of hydration in the internal VSD cavity, a feature favoring the occurrence of ω-currents. Free energy and kinetics calculations of sodium permeation through the native and mutated VSD indicate as more favorable the formation of a cationic current in the latter. Overall, our simulations establish a mechanistic linkage between genetic variations and their physiological outcome, by providing a computational description that includes both thermodynamic and kinetic features of ion permeation associated with ω-currents.
Highlights
IntroductionActivation of ion conductance in voltage-gated ion channels (VGICs) is regulated by four peripheral voltage sensing domains (VSDs),[1,2] each made of four transmembrane helices (S1 to S4) and linked to the central pore domain (PD).[3−8] In response to variations of the transmembrane potential, positively charged amino acids on the S4 helix (typically arginines) change position, inducing a conformational transition in the VSD9,10 that is transmitted to the pore domains (PDs) and triggers its opening to ion translocation.[11−16] The movement of the charged residues generates an electric current called the gating current.[17]In functional channels, conduction through the VSD is impeded by a hydrophobic constriction separating the extracellular from the intracellular space, and neither water nor ions can pass through it
Activation of ion conductance in voltage-gated ion channels (VGICs) is regulated by four peripheral voltage sensing domains (VSDs),[1,2] each made of four transmembrane helices (S1 to S4) and linked to the central pore domain (PD).[3−8] In response to variations of the transmembrane potential, positively charged amino acids on the S4 helix change position, inducing a conformational transition in the VSD9,10 that is transmitted to the PD and triggers its opening to ion translocation.[11−16] The movement of the charged residues generates an electric current called the gating current.[17]
We used a combination of molecular modeling and molecular dynamics (MD) simulations to reconstruct the structural features of a mutated Kv7.2 VSD associated with the formation of ωcurrents at depolarized potentials (R207Q)
Summary
Activation of ion conductance in voltage-gated ion channels (VGICs) is regulated by four peripheral voltage sensing domains (VSDs),[1,2] each made of four transmembrane helices (S1 to S4) and linked to the central pore domain (PD).[3−8] In response to variations of the transmembrane potential, positively charged amino acids on the S4 helix (typically arginines) change position, inducing a conformational transition in the VSD9,10 that is transmitted to the PD and triggers its opening to ion translocation.[11−16] The movement of the charged residues generates an electric current called the gating current.[17]In functional channels, conduction through the VSD is impeded by a hydrophobic constriction separating the extracellular from the intracellular space, and neither water nor ions can pass through it. Mutations in the VSD can result in an altered response to transmembrane voltage and changes in channel gating, a common mechanism of genetically inherited channelopathies.[18−24] When mutations affect the S4 arginines, anomalous currents (named gating pore currents, ω-pore currents, or ω-currents) have been recorded for various channels[25−27] and attributed to protons[28,29] or cations[30−32] passing directly through the VSD In addition to their pathophysiological relevance, ω-currents have been exploited to dissect the structural features of VSDs,[33,34] since they are related to mutations of residues that stabilize the sensor in the different active or resting states. Other Kv7.2 pathogenic mutations (R213W, R213Q, D212G, E140Q) were described to impair the channel functional activity, supposedly
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