Molecular mechanism for depolarization-induced modulation of Kv channel closure

Voltage-dependent potassium (Kv) channels provide the repolarizing power that shapes the action potential duration and helps control the firing frequency of neurons. The K⁺ permeation through the channel pore is controlled by an intracellularly located bundle-crossing (BC) gate that communicates with the voltage-sensing domains (VSDs). During prolonged membrane depolarizations, most Kv channels display C-type inactivation that halts K⁺ conduction through constriction of the K⁺ selectivity filter. Besides triggering C-type inactivation, we show that in Shaker and Kv1.2 channels (expressed in Xenopus laevis oocytes), prolonged membrane depolarizations also slow down the kinetics of VSD deactivation and BC gate closure during the subsequent membrane repolarization. Measurements of deactivating gating currents (reporting VSD movement) and ionic currents (BC gate status) showed that the kinetics of both slowed down in two distinct phases with increasing duration of the depolarizing prepulse. The biphasic slowing in VSD deactivation and BC gate closure was strongly correlated in time and magnitude. Simultaneous recordings of ionic currents and fluorescence from a probe tracking VSD movement in Shaker directly demonstrated that both processes were synchronized. Whereas the first slowing originates from a stabilization imposed by BC gate opening, the subsequent slowing reflects the rearrangement of the VSD toward its relaxed state (relaxation). The VSD relaxation was observed in the Ciona intestinalis voltage-sensitive phosphatase and in its isolated VSD. Collectively, our results show that the VSD relaxation is not kinetically related to C-type inactivation and is an intrinsic property of the VSD. We propose VSD relaxation as a general mechanism for depolarization-induced slowing of BC gate closure that may enable Kv1.2 channels to modulate the firing frequency of neurons based on the depolarization history.

Being flexible: the voltage-controllable activation gate of kv channels

Kv channels form voltage-dependent potassium selective pores in the outer cell membrane and are composed out of four α-subunits, each having six membrane-spanning α-helices (S1–S6). The α-subunits tetramerize such that the S5–S6 pore domains co-assemble into a centrally located K⁺ pore which is surrounded by four operational voltage-sensing domains (VSD) that are each formed by the S1–S4 segments. Consequently, each subunit is capable of responding to changes in membrane potential and dictates whether the pore should be conductive or not. K⁺ permeation through the pore can be sealed off by two separate gates in series: (a) at the inner S6 bundle crossing (BC gate) and (b) at the level of the selectivity filter (SF gate) located at the extracellular entrance of the pore. Within the last years a general consensus emerged that a direct communication between the S4S5-linker and the bottom part of S6 (S6_c) constitutes the coupling with the VSD thus making the BC gate the main voltage-controllable activation gate. While the BC gate listens to the VSD, the SF changes its conformation depending on the status of the BC gate. Through the eyes of an entering K⁺ ion, the operation of the BC gate apparatus can be compared with the iris-like motion of the diaphragm from a camera whereby its diameter widens. Two main gating motions have been proposed to create this BC gate widening: (1) tilting of the helix whereby the S6 converts from a straight α-helix to a tilted one or (2) swiveling of the S6_c whereby the S6 remains bent. Such motions require a flexible hinge that decouples the pre- and post-hinge segment. Roughly at the middle of the S6 there exists a highly conserved glycine residue and a tandem proline motif that seem to fulfill the role of a gating hinge which allows for tilting/swiveling/rotations of the post-hinge S6 segment. In this review we delineate our current view on the operation of the BC gate for controlling K⁺ permeation in Kv channels.

Dual effect of phosphatidylinositol (4,5)-bisphosphate PIP(2) on Shaker K(+) [corrected] channels

Phosphatidylinositol (4,5)-bisphosphate (PIP(2)) is a phospholipid of the plasma membrane that has been shown to be a key regulator of several ion channels. Functional studies and more recently structural studies of Kir channels have revealed the major impact of PIP(2) on the open state stabilization. A similar effect of PIP(2) on the delayed rectifiers Kv7.1 and Kv11.1, two voltage-gated K(+) channels, has been suggested, but the molecular mechanism remains elusive and nothing is known on PIP(2) effect on other Kv such as those of the Shaker family. By combining giant-patch ionic and gating current recordings in COS-7 cells, and voltage-clamp fluorimetry in Xenopus oocytes, both heterologously expressing the voltage-dependent Shaker channel, we show that PIP(2) exerts 1) a gain-of-function effect on the maximal current amplitude, consistent with a stabilization of the open state and 2) a loss-of-function effect by positive-shifting the activation voltage dependence, most likely through a direct effect on the voltage sensor movement, as illustrated by molecular dynamics simulations.

Kv3 channels contribute to the delayed rectifier current in small cultured mouse dorsal root ganglion neurons

Delayed rectifier voltage-gated K(+) (K(V)) channels are important determinants of neuronal excitability. However, the large number of K(V) subunits poses a major challenge to establish the molecular composition of the native neuronal K(+) currents. A large part (∼60%) of the delayed rectifier current (I(K)) in small mouse dorsal root ganglion (DRG) neurons has been shown to be carried by both homotetrameric K(V)2.1 and heterotetrameric channels of K(V)2 subunits with silent K(V) subunits (K(V)S), while a contribution of K(V)1 channels has also been demonstrated. Because K(V)3 subunits also generate delayed rectifier currents, we investigated the contribution of K(V)3 subunits to I(K) in small mouse DRG neurons. After stromatoxin (ScTx) pretreatment to block the K(V)2-containing component, application of 1 mM TEA caused significant additional block, indicating that the ScTx-insensitive part of I(K) could include K(V)1, K(V)3, and/or M-current channels (KCNQ2/3). Combining ScTx and dendrotoxin confirmed a relevant contribution of K(V)2 and K(V)2/K(V)S, and K(V)1 subunits to I(K) in small mouse DRG neurons. After application of these toxins, a significant TEA-sensitive current (∼19% of total I(K)) remained with biophysical properties that corresponded to those of K(V)3 currents obtained in expression systems. Using RT-PCR, we detected K(V)3.1-3 mRNA in DRG neurons. Furthermore, Western blot and immunocytochemistry using K(V)3.1-specific antibodies confirmed the presence of K(V)3.1 in cultured DRG neurons. These biophysical, pharmacological, and molecular results demonstrate a relevant contribution (∼19%) of K(V)3-containing channels to I(K) in small mouse DRG neurons, supporting a substantial role for K(V)3 subunits in these neurons.

The electrically silent Kv6.4 subunit confers hyperpolarized gating charge movement in Kv2.1/Kv6.4 heterotetrameric channels

The voltage-gated K(+) (Kv) channel subunit Kv6.4 does not form functional homotetrameric channels but co-assembles with Kv2.1 to form functional Kv2.1/Kv6.4 heterotetrameric channels. Compared to Kv2.1 homotetramers, Kv6.4 exerts a ~40 mV hyperpolarizing shift in the voltage-dependence of Kv2.1/Kv6.4 channel inactivation, without a significant effect on activation gating. However, the underlying mechanism of this Kv6.4-induced modulation of Kv2.1 channel inactivation, and whether the Kv6.4 subunit participates in the voltage-dependent gating of heterotetrameric channels is not well understood. Here we report distinct gating charge movement of Kv2.1/Kv6.4 heterotetrameric channels, compared to Kv2.1 homotetramers, as revealed by gating current recordings from mammalian cells expressing these channels. The gating charge movement of Kv2.1/Kv6.4 heterotetrameric channels displayed an extra component around the physiological K(+) equilibrium potential, characterized by a second sigmoidal relationship of the voltage-dependence of gating charge movement. This distinct gating charge displacement reflects movement of the Kv6.4 voltage-sensing domain and has a voltage-dependency that matches the hyperpolarizing shift in Kv2.1/Kv6.4 channel inactivation. These results provide a mechanistic basis for the modulation of Kv2.1 channel inactivation gating kinetics by silent Kv6.4 subunits.

Electrically Silent Kv Subunits: Their Molecular and Functional Characteristics

Electrically silent voltage-gated potassium (KvS) α-subunits do not form homotetramers but heterotetramerize with Kv2 subunits, generating functional Kv2/KvS channel complexes in which the KvS subunits modulate the Kv2 current. This poses intriguing questions into the molecular mechanisms by which these KvS subunits cannot form functional homotetramers, why they only interact with Kv2 subunits, and how they modulate the Kv2 current.

The S4-S5 linker of KCNQ1 channels forms a structural scaffold with the S6 segment controlling gate closure

In vivo, KCNQ1 α-subunits associate with the β-subunit KCNE1 to generate the slowly activating cardiac potassium current (I(Ks)). Structurally, they share their topology with other Kv channels and consist out of six transmembrane helices (S1-S6) with the S1-S4 segments forming the voltage-sensing domain (VSD). The opening or closure of the intracellular channel gate, which localizes at the bottom of the S6 segment, is directly controlled by the movement of the VSD via an electromechanical coupling. In other Kv channels, this electromechanical coupling is realized by an interaction between the S4-S5 linker (S4S5(L)) and the C-terminal end of S6 (S6(T)). Previously we reported that substitutions for Leu(353) in S6(T) resulted in channels that failed to close completely. Closure could be incomplete because Leu(353) itself is the pore-occluding residue of the channel gate or because of a distorted electromechanical coupling. To resolve this and to address the role of S4S5(L) in KCNQ1 channel gating, we performed an alanine/tryptophan substitution scan of S4S5(L). The residues with a "high impact" on channel gating (when mutated) clustered on one side of the S4S5(L) α-helix. Hence, this side of S4S5(L) most likely contributes to the electromechanical coupling and finds its residue counterparts in S6(T). Accordingly, substitutions for Val(254) resulted in channels that were partially constitutively open and the ability to close completely was rescued by combination with substitutions for Leu(353) in S6(T). Double mutant cycle analysis supported this cross-talk indicating that both residues come in close contact and stabilize the closed state of the channel.

KCNQ1 channels voltage dependence through a voltage-dependent binding of the S4-S5 linker to the pore domain

Voltage-dependent potassium (Kv) channels are tetramers of six transmembrane domain (S1-S6) proteins. Crystallographic data demonstrate that the tetrameric pore (S5-S6) is surrounded by four voltage sensor domains (S1-S4). One key question remains: how do voltage sensors (S4) regulate pore gating? Previous mutagenesis data obtained on the Kv channel KCNQ1 highlighted the critical role of specific residues in both the S4-S5 linker (S4S5(L)) and S6 C terminus (S6(T)). From these data, we hypothesized that S4S5(L) behaves like a ligand specifically interacting with S6(T) and stabilizing the closed state. To test this hypothesis, we designed plasmid-encoded peptides corresponding to portions of S4S5(L) and S6(T) of the voltage-gated potassium channel KCNQ1 and evaluated their effects on the channel activity in the presence and absence of the ancillary subunit KCNE1. We showed that S4S5(L) peptides inhibit KCNQ1, in a reversible and state-dependent manner. S4S5(L) peptides also inhibited a voltage-independent KCNQ1 mutant. This inhibition was competitively prevented by a peptide mimicking S6(T), consistent with S4S5(L) binding to S6(T). Val(254) in S4S5(L) is known to contact Leu(353) in S6(T) when the channel is closed, and mutations of these residues alter the coupling between the two regions. The same mutations introduced in peptides altered their effects, further confirming S4S5(L) binding to S6(T). Our results suggest a mechanistic model in which S4S5(L) acts as a voltage-dependent ligand bound to its receptor on S6 at rest. This interaction locks the channel in a closed state. Upon plasma membrane depolarization, S4 pulls S4S5(L) away from S6(T), allowing channel opening.

The rate-dependent biophysical properties of the LQT1 H258R mutant are counteracted by a dominant negative effect on channel trafficking

The long QT syndrome (LQTS) is a cardiac disorder caused by a prolonged ventricular repolarization. The co-assembly of the pore-forming human KCNQ1 alpha-subunits with the modulating hKCNE1 beta-subunits generates I(Ks)in vivo, explaining why mutations in the hKCNQ1 gene underlie the LQT1 form of congenital LQT. Here we describe the functional defects of the LQT1 mutation H258R located in the S4-S5 linker, a segment important for channel gating. Mutant subunits with this arginine substitution generated no or barely detectable currents in a homotetrameric condition, but did generate I(Ks)-like currents in association with hKCNE1. Compared to the WT hKCNQ1/hKCNE1 complex, the H258R/hKCNE1 complex displayed accelerated activation kinetics, slowed channel closure and a hyperpolarizing shift of the voltage-dependence of activation, thus predicting an increased K(+) current. However, current density analysis combined with subcellular localization indicated that the H258R subunit exerted a dominant negative effect on channel trafficking to the plasma membrane. The co-expression hKCNQ1/H258R/hKCNE1, mimicking the heterozygous state of a patient, displayed similar properties. During repetitive stimulation the mutant yielded more current compared to WT at 1 Hz but this effect was counteracted by the trafficking defect at faster frequencies. These rate-dependent effects may be relevant given the larger contribution of I(Ks) to the "repolarization reserve" at higher action potential rates. The combination of complex kinetics that counteract the trafficking problem represents a particular mechanism underlying LQT1.

Conserved negative charges in the N-terminal tetramerization domain mediate efficient assembly of Kv2.1 and Kv2.1/Kv6.4 channels

Voltage-gated potassium (Kv) channels are transmembrane tetramers of individual alpha-subunits. Eight different Shaker-related Kv subfamilies have been identified in which the tetramerization domain T1, located on the intracellular N terminus, facilitates and controls the assembly of both homo- and heterotetrameric channels. Only the Kv2 alpha-subunits are able to form heterotetramers with members of the silent Kv subfamilies (Kv5, Kv6, Kv8, and Kv9). The T1 domain contains two subdomains, A and B box, which presumably determine subfamily specificity by preventing incompatible subunits to assemble. In contrast, little is known about the involvement of the A/B linker sequence. Both Kv2 and silent Kv subfamilies contain a fully conserved and negatively charged sequence (CDD) in this linker that is lacking in the other subfamilies. Neutralizing these aspartates in Kv2.1 by mutating them to alanines did not affect the gating properties, but reduced the current density moderately. However, charge reversal arginine substitutions strongly reduced the current density of these homotetrameric mutant Kv2.1 channels and immunocytochemistry confirmed the reduced expression at the plasma membrane. Förster resonance energy transfer measurements using confocal microscopy showed that the latter was not due to impaired trafficking, but to a failure to assemble the tetramer. This was further confirmed with co-immunoprecipitation experiments. The corresponding arginine substitution in Kv6.4 prevented its heterotetrameric interaction with Kv2.1. These results indicate that these aspartates (especially the first one) in the A/B box linker of the T1 domain are required for efficient assembly of both homotetrameric Kv2.1 and heterotetrameric Kv2.1/silent Kv6.4 channels.

Kv2.1 and silent Kv subunits underlie the delayed rectifier K+ current in cultured small mouse DRG neurons

Silent voltage-gated K(+) (K(v)) subunits interact with K(v)2 subunits and primarily modulate the voltage dependence of inactivation of these heterotetrameric channels. Both K(v)2 and silent K(v) subunits are expressed in the mammalian nervous system, but little is known about their expression and function in sensory neurons. This study reports the presence of K(v)2.1, K(v)2.2, and silent subunit K(v)6.1, K(v)8.1, K(v)9.1, K(v)9.2, and K(v)9.3 mRNA in mouse dorsal root ganglia (DRG). Immunocytochemistry confirmed the protein expression of K(v)2.x and K(v)9.x subunits in cultured small DRG neurons. To investigate if K(v)2 and silent K(v) subunits are underlying the delayed rectifier K(+) current (I(K)) in these neurons, K(v)2-mediated currents were isolated by the extracellular application of rStromatoxin-1 (ScTx) or by the intracellular application of K(v)2 antibodies. Both ScTx- and anti-K(v)2.1-sensitive currents displayed two components in their voltage dependence of inactivation. Together, both components accounted for approximately two-thirds of I(K). A comparison with results obtained in heterologous expression systems suggests that one component reflects homotetrameric K(v)2.1 channels, whereas the other component represents heterotetrameric K(v)2.1/silent K(v) channels. These observations support a physiological role for silent K(v) subunits in small DRG neurons.

Kv channel gating requires a compatible S4-S5 linker and bottom part of S6, constrained by non-interacting residues

Voltage-dependent K⁺ channels transfer the voltage sensor movement into gate opening or closure through an electromechanical coupling. To test functionally whether an interaction between the S4-S5 linker (L45) and the cytoplasmic end of S6 (S6_T) constitutes this coupling, the L45 in hKv1.5 was replaced by corresponding hKv2.1 sequence. This exchange was not tolerated but could be rescued by also swapping S6_T. Exchanging both L45 and S6_T transferred hKv2.1 kinetics to an hKv1.5 background while preserving the voltage dependence. A one-by-one residue substitution scan of L45 and S6_T in hKv1.5 further shows that S6_T needs to be α-helical and forms a “crevice” in which residues I422 and T426 of L45 reside. These residues transfer the mechanical energy onto the S6_T crevice, whereas other residues in S6_T and L45 that are not involved in the interaction maintain the correct structure of the coupling.

A Kv channel with an altered activation gate sequence displays both “fast” and “slow” activation kinetics

The Kv1–4 families of K+ channels contain a tandem proline motif (P XP) in the S6 helix that is crucial for channel gating. In human Kv1.5, replacing the first proline by an alanine resulted in a nonfunctional channel. This mutant was rescued by introducing another proline at a nearby position, changing the sequence into AVPP. This resulted in a channel that activated quickly (ms range) upon the first depolarization. However, thereafter, the channel became trapped in another gating mode that was characterized by slow activation kinetics (s range) with a shallow voltage dependence. The switch in gating mode was observed even with very short depolarization steps, but recovery to the initial “fast” mode was extremely slow. Computational modeling suggested that switching occurred during channel deactivation. To test the effect of the altered P XP sequence on the mobility of the S6 helix, we used molecular dynamics simulations of the isolated S6 domain of wild type (WT) and mutants starting from either a closed or open conformation. The WT S6 helix displayed movements around the P XP region with simulations starting from either state. However, the S6 with a AVPP sequence displayed flexibility only when started from the closed conformation and was rigid when started from the open state. These results indicate that the region around the P XP motif may serve as a “hinge” and that changing the sequence to AVPP results in channels that deactivate to a state with an alternate configuration that renders them “reluctant” to open subsequently.

Gambierol, a toxin produced by the dinoflagellate Gambierdiscus toxicus, is a potent blocker of voltage-gated potassium channels

In this study, we pharmacologically characterized gambierol, a marine polycyclic ether toxin which is produced by the dinoflagellate Gambierdiscus toxicus. Besides several other polycyclic ether toxins like ciguatoxins, this scarcely studied toxin is one of the compounds that may be responsible for ciguatera fish poisoning (CFP). Unfortunately, the biological target(s) that underlies CFP is still partly unknown. Today, ciguatoxins are described to specifically activate voltage-gated sodium channels by interacting with their receptor site 5. But some dispute about the role of gambierol in the CFP story shows up: some describe voltage-gated sodium channels as the target, while others pinpoint voltage-gated potassium channels as targets. Since gambierol was never tested on isolated ion channels before, it was subjected in this work to extensive screening on a panel of 17 ion channels: nine cloned voltage-gated ion channels (mammalian Na(v)1.1-Na(v)1.8 and insect Para) and eight cloned voltage-gated potassium channels (mammalian K(v)1.1-K(v)1.6, hERG and insect ShakerIR) expressed in Xenopus laevis oocytes using two-electrode voltage-clamp technique. All tested sodium channel subtypes are insensitive to gambierol concentrations up to 10 microM. In contrast, K(v)1.2 is the most sensitive voltage-gated potassium channel subtype with almost full block (>97%) and an half maximal inhibitory concentration (IC(50)) of 34.5 nM. To the best of our knowledge, this is the first study where the selectivity of gambierol is tested on isolated voltage-gated ion channels. Therefore, these results lead to a better understanding of gambierol and its possible role in CFP and they may also be useful in the development of more effective treatments.

Role of the S6 C-terminus in KCNQ1 channel gating

Co-assembly of KCNQ1 alpha-subunits with KCNE1 beta-subunits results in the channel complex underlying the cardiac IKs current in vivo. Like other voltage-gated K+ channels, KCNQ1 has a tetrameric configuration. The S6 segment of each subunit lines the ion channel pore with the lower part forming the activation gate. To determine residues involved in protein-protein interactions in the C-terminal part of S6 (S6T), alanine and tryptophan perturbation scans were performed from residue 348-362 in the KCNQ1 channel. Several residues were identified to be relevant in channel gating, as substitutions affected the activation and/or deactivation process. Some mutations (F351A and V355W) drastically altered the gating characteristics of the resultant KCNQ1 channel, to the point of mimicking the IKs current. Furthermore, mutagenesis of residue L353 to an alanine or a charged residue impaired normal channel closure upon hyperpolarization, generating a constitutively open phenotype. This indicates that the L353 residue is essential for stabilizing the closed conformation of the channel gate. These findings together with the identification of several LQT1 mutations in the S6 C-terminus of KCNQ1 underscore the relevance of this region in KCNQ1 and IKs channel gating.

The aromatic cluster in KCHIP1b affects Kv4 inactivation gating

The KChIP1b splice variant has been shown to induce slow recovery from inactivation for Kv4.2 whereas KChIP1a enhanced the recovery. Both splice variants differ only by the insertion of the exon1b, rich in aromatic residues (5/11). We analysed in detail the modifications of Kv4.2 gating induced by the KChIP1b splice variant and the role for the aromatic cluster in KChIP1b in inducing these changes. By substituting alanine for the aromatic residues individually or in combination, we could convert the KChIP1b recovery behaviour into that of KChIP1a. The replacement of one or two aromatic residues resulted in a partial restitution of the KChIP1a recovery behaviour. When three aromatic residues were replaced in the exon1b, the recovery from inactivation was fast with time constants that were similar to those obtained with KChIP1a. Moreover, similar findings were observed for closed state inactivation and for the voltage dependence of inactivation. Thus, reduction of the side chain bulkiness in exon1b resulted in the conversion of the KChIP1b phenotype into the KChIP1a phenotype. These results indicate that the aromatic cluster in exon1b modulates the transitions towards and from the closed inactivated states and the steady state distribution over the respective states.