KChIP2b modulates the affinity and use-dependent block of Kv4.3 by nifedipine

Ion channel traffic jams: the significance of trafficking deficiency in long QT syndrome

A well-balanced ion channel trafficking machinery is paramount for the normal electromechanical function of the heart. Ion channel variants and many drugs can alter the cardiac action potential and lead to arrhythmias by interfering with mechanisms like ion channel synthesis, trafficking, gating, permeation, and recycling. A case in point is the Long QT syndrome (LQTS), a highly arrhythmogenic disease characterized by an abnormally prolonged QT interval on ECG produced by variants and drugs that interfere with the action potential. Disruption of ion channel trafficking is one of the main sources of LQTS. We review some molecular pathways and mechanisms involved in cardiac ion channel trafficking. We highlight the importance of channelosomes and other macromolecular complexes in helping to maintain normal cardiac electrical function, and the defects that prolong the QT interval as a consequence of variants or the effect of drugs. We examine the concept of "interactome mapping" and illustrate by example the multiple protein-protein interactions an ion channel may undergo throughout its lifetime. We also comment on how mapping the interactomes of the different cardiac ion channels may help advance research into LQTS and other cardiac diseases. Finally, we discuss how using human induced pluripotent stem cell technology to model ion channel trafficking and its defects may help accelerate drug discovery toward preventing life-threatening arrhythmias. Advancements in understanding ion channel trafficking and channelosome complexities are needed to find novel therapeutic targets, predict drug interactions, and enhance the overall management and treatment of LQTS patients.

NS8593 inhibits Ca2+ permeant channels reversing mouse airway smooth muscle contraction

AIMS

This study focused on investigating whether NS8593 reverses airway smooth muscle (ASM) contraction and the underlying mechanism.

MAIN METHODS

ASM contraction in mouse tracheal rings and lung slices was measured. Currents mediated by voltage dependent Ca²⁺ channels (VDCCs) and ACH-activated channels were measured using the whole-cell patch-clamp technique in single tracheal smooth muscle cells (TSMCs). Intracellular Ca²⁺ level and cell length were measured using an LSM 700 laser confocal microscope and a Zen 2010 software. Mouse respiratory system resistance (Rrs) was assessed using a FlexiVent FX system.

KEY FINDINGS

High K⁺ (80 mM K⁺) and ACH induced ASM contraction in mouse tracheal rings and lung slices, which was partially relaxed by nifedipine (blocker of L-type VDCCs, LVDCCs), YM-58483 (blocker of store-operated Ca²⁺ entry (SOCE), transient receptor potential C3 (TRPC3) and TRPC5 channels), respectively. However, the contraction was completely reversed by NS8593, whereas, slightly relaxed by formoterol. ACH activated inward currents, which displayed linear and reversed around 0 mV, indicating the currents were mediated by non-selective cation channels (NSCCs). Moreover, these currents were blocked by YM-58483. In addition, such currents were abolished by NS8593, implicating that NS8593 inhibits the same channels. Besides, NS8593 inhibited increases of intracellular Ca²⁺ and the associated cell shortening. Finally, NS8593 inhibited ACH-induced increases of mouse respirator system resistance (Rrs).

SIGNIFICANCE

Our results indicate that NS8593 inhibits LVDCCs and NSCCs, resulting in decreases of intracellular Ca²⁺ and then leading to ASM relaxation. These data suggest that NS8593 might be a new bronchodilator.

β Subunits Control the Effects of Human Kv4.3 Potassium Channel Phosphorylation

The transient outward K⁺ current, I_to, activates early in the cardiac myocyte action potential, to begin repolarization. Human I_to is generated primarily by two Kv4.3 potassium channel α subunit splice variants (Kv4.3L and Kv4.3S) that diverge only by a C-terminal, membrane-proximal, 19-residue stretch unique to Kv4.3L. Protein kinase C (PKC) phosphorylation of threonine 504 within the Kv4.3L-specific 19-residues mediates α-adrenergic inhibition of I_to in human heart. Kv4.3 is regulated in human heart by various β subunits, including cytosolic KChIP2b and transmembrane KCNEs, yet their impact on the functional effects of human Kv4.3 phosphorylation has not been reported. Here, this gap in knowledge was addressed using human Kv4.3 splice variants, T504 mutants, and human β subunits. Subunits were co-expressed in Xenopus laevis oocytes and analyzed by two-electrode voltage-clamp, using phorbol 12-myristate 13-acetate (PMA) to stimulate PKC. Unexpectedly, KChIP2b removed the inhibitory effect of PKC on Kv4.3L (but not Kv4.3L threonine phosphorylation by PKC per-se), while co-expression with KCNE2, but not KCNE4, restored PKC-dependent inhibition of Kv4.3L-KChIP2b to quantitatively resemble previously reported effects of α-adrenergic modulation of human ventricular I_to. In addition, PKC accelerated recovery from inactivation of Kv4.3L-KChIP2b channels and, interestingly, of both Kv4.3L and Kv4.3S alone. Thus, β subunits regulate the response of human Kv4.3 to PKC phosphorylation and provide a potential mechanism for modifying the response of I_to to α-adrenergic regulation in vivo.

Use-dependent inhibition of glycine-activated chloride current in rat neurons by β-amyloid peptide pretreated with hexafluoroisopropanol

Hexafluoroisopropanol (HFIP) is a nonpolar organic solvent that is often used to prepare β-amyloid peptide (Aβ) samples. In this work, we compare the effects of two different species derived from synthetic Aβ1-42 and prepared without HFIP (Aβ) or using HFIP (Aβ/HFIP) on the glycine-activated chloride current (IGly). The experiments were conducted on the pyramidal neurons isolated from CA3 region of rat hippocampus. Transmembrane currents were recorded using a conventional patch-clamp technique in the whole-cell configuration. The IGly was induced by a step application of the agonist for 600 ms through glass capillary. Aβ or Aβ/HFIP was coapplied with glycine. The effects of the two species of the peptide have similar and distinctive features. Both substances caused a reduction in the peak amplitude and an acceleration of desensitization of the IGly. At the same time, the effect of Aβ/HFIP was found to develop and recover more slowly and required several repeated applications for its saturation (use dependence). The effect of Aβ/HFIP was voltage independent and equally pronounced at negative and positive membrane potentials. First, our results confirm that HFIP pretreatment may influence the properties of Aβ. Second, new information on the glycine receptor ability to interact with drugs in use-dependent mode was obtained.

Hormones and sex differences: changes in cardiac electrophysiology with pregnancy

Disruption of cardiac electrical activity resulting in palpitations and syncope is often an early symptom of pregnancy. Pregnancy is a time of dramatic and dynamic physiological and hormonal changes during which numerous demands are placed on the heart. These changes result in electrical remodelling which can be detected as changes in the electrocardiogram (ECG). This gestational remodelling is a very under-researched area. There are no systematic large studies powered to determine changes in the ECG from pre-pregnancy, through gestation, and into the postpartum period. The large variability between patients and the dynamic nature of pregnancy hampers interpretation of smaller studies, but some facts are consistent. Gestational cardiac hypertrophy and a physical shift of the heart contribute to changes in the ECG. There are also electrical changes such as an increased heart rate and lengthening of the QT interval. There is an increased susceptibility to arrhythmias during pregnancy and the postpartum period. Some changes in the ECG are clearly the result of changes in ion channel expression and behaviour, but little is known about the ionic basis for this electrical remodelling. Most information comes from animal models, and implicates changes in the delayed-rectifier channels. However, it is likely that there are additional roles for sodium channels as well as changes in calcium homoeostasis. The changes in the electrical profile of the heart during pregnancy and the postpartum period have clear implications for the safety of pregnant women, but the field remains relatively undeveloped.

The Inhibitory Effects of Ca2+ Channel Blocker Nifedipine on Rat Kv2.1 Potassium Channels

It is well documented that nifedipine, a commonly used dihydropyridine Ca²⁺ channel blocker, has also significant interactions with voltage-gated K⁺ (Kv) channels. But to date, little is known whether nifedipine exerted an action on Kv2.1 channels, a member of the Shab subfamily with slow inactivation. In the present study, we explored the effects of nifedipine on rat Kv2.1 channels expressed in HEK293 cells. Data from whole-cell recording showed that nifedipine substantially reduced Kv2.1 currents with the IC₅₀ value of 37.5 ± 5.7 μM and delayed the time course of activation without effects on the activation curve. Moreover, this drug also significantly shortened the duration of inactivation and deactivation of Kv2.1 currents in a voltage-dependent manner. Interestingly, the half-maximum inactivation potential (V_1/2) of Kv2.1 currents was -11.4 ± 0.9 mV in control and became -38.5 ± 0.4 mV after application of 50 μM nifedipine. The large hyperpolarizing shift (27 mV) of the inactivation curve has not been reported previously and may result in more inactivation for outward delayed rectifier K⁺ currents mediated by Kv2.1 channels at repolarization phases. The Y380R mutant significantly increased the binding affinity of nifedipine to Kv2.1 channels, suggesting an interaction of nifedipine with the outer mouth region of this channel. The data present here will be helpful to understand the diverse effects exerted by nifedipine on various Kv channels.

Raloxifene inhibits cloned Kv4.3 channels in an estrogen receptor-independent manner

Raloxifene is widely used for the treatment and prevention of postmenopausal osteoporosis. We examined the effects of raloxifene on the Kv4.3 currents expressed in Chinese hamster ovary (CHO) cells using the whole-cell patch-clamp technique and on the long-term modulation of Kv4.3 messenger RNA (mRNA) by real-time PCR analysis. Raloxifene decreased the Kv4.3 currents with an IC50 of 2.0 μM and accelerated the inactivation and activation kinetics in a concentration-dependent manner. The inhibitory effects of raloxifene on Kv4.3 were time-dependent: the association and dissociation rate constants for raloxifene were 9.5 μM(-1) s(-1) and 23.0 s(-1), respectively. The inhibition by raloxifene was voltage-dependent (δ = 0.13). Raloxifene shifted the steady-state inactivation curves in a hyperpolarizing direction and accelerated the closed-state inactivation of Kv4.3. Raloxifene slowed the time course of recovery from inactivation, thus producing a use-dependent inhibition of Kv4.3. β-Estradiol and tamoxifen had little effect on Kv4.3. A preincubation of ICI 182,780, an estrogen receptor antagonist, for 1 h had no effect on the inhibitory effect of raloxifene on Kv4.3. The metabolites of raloxifene, raloxifene-4'-glucuronide and raloxifene-6'-glucuronide, had little or no effect on Kv4.3. Coexpression of KChIP2 subunits did not alter the drug potency and steady-state inactivation of Kv4.3 channels. Long-term exposure to raloxifene (24 h) significantly decreased the expression level of Kv4.3 mRNA. This effect was not abolished by the coincubation with ICI 182,780. Raloxifene inhibited Kv4.3 channels by interacting with their open state during depolarization and with the closed state at subthreshold potentials. This effect was not mediated via an estrogen receptor.

Expression of kv4.3 voltage-gated potassium channels in rat gonadotrophin-releasing hormone (GnRH) neurons during the estrous cycle

Regular and timely electrical activity of gonadotrophin-releasing hormone (GnRH) neurons accompanies the pulsatile release of GnRH that plays a central role in regulating fertility. Although transient outward A-type currents (I(A)) have been electrophysiologically identified in GnRH neurons, the molecular identity of the channels that underlie these currents are unknown. Several families of voltage-gated potassium channels can underlie I(A). However, the biophysical properties of I(A) described in previous electrophysiological studies are strongly characteristic of members of the Kv4 family of voltage-gated channels. We, therefore, sought to determine the presence of Kv4 channels in GnRH neurons. We used reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blot analysis to determine whether Kv4 messenger RNA (mRNA) and protein are present in the rat medial preoptic area (MPOA) and median eminence (ME). We used double-label immunohistochemistry to determine whether Kv4 colocalized with GnRH cell bodies in the MPOA and GnRH axons in the ME. Kv4.3 channels co-localized with GnRH in the MPOA but not in the ME. Neither Kv4.2 nor Kv4.1 co-localized with GnRH in either the MPOA or the ME. The electrical activity of GnRH neurons changes dramatically during the estrous cycle. We, therefore, studied the change in Kv4.3 expression in GnRH neurons during the estrous cycle. In the estrus phase, 58.05% of GnRH neurons expressed Kv4.3 compared to 74.48% in diestrus-proestrus rats (P < .05). Our data suggest that Kv4.3 is the major molecular component of I(A) in GnRH neurons, and furthermore that the expression of Kv4.3 changes significantly during the rat estrous cycle.

Effect of the I(to) activator NS5806 on cloned K(V)4 channels depends on the accessory protein KChIP2

BACKGROUND AND PURPOSE

The compound NS5806 increases the transient outward current (I(to)) in canine ventricular cardiomyocytes and slows current decay. In human and canine ventricle, I(to) is thought to be mediated by K(V)4.3 and various ancillary proteins, yet, the exact subunit composition of I(to) channels is still debated. Here we characterize the effect of NS5806 on heterologously expressed putative I(to) channel subunits and other potassium channels.

EXPERIMENTAL APPROACH

Cloned K(V)4 channels were co-expressed with KChIP2, DPP6, DPP10, KCNE2, KCNE3 and K(V)1.4 in Xenopus laevis oocytes or CHO-K1 cells.

KEY RESULTS

NS5806 increased K(V)4.3/KChIP2 peak current amplitudes with an EC(50) of 5.3 +/- 1.5microM and significantly slowed current decay. KCNE2, KCNE3, DPP6 and DPP10 modulated K(V)4.3 currents and the response to NS5806, but current decay was slowed only in complexes containing KChIP2. The effect of NS5806 on K(V)4.2 was similar to that on K(V)4.3, and current decay was only slowed in presence of KChIP2. However, for K(V)4.1, the slowing of current decay by NS5806 was independent of KChIP2. K(V)1.4 was strongly inhibited by 10 microM NS5806 and K(V)1.5 was inhibited to a smaller extent. Effects of NS5806 on kinetics of currents generated by K(V)4.3/KChIP2/DPP6 with K(V)1.4 in oocytes could reproduce those on cardiac I(to) in canine ventricular myocytes. K(V)7.1, K(V)11.1 and K(ir)2 currents were unaffected by NS5806.

CONCLUSION AND IMPLICATIONS

NS5806 modulated K(V)4 channel gating depending on the presence of KChIP2, suggesting that NS5806 can potentially be used to address the molecular composition as well as the physiological role of cardiac I(to).

S3b amino acid substitutions and ancillary subunits alter the affinity of Heteropoda venatoria toxin 2 for Kv4.3

Heteropoda venatoria toxin 2 (HpTx2) is an inhibitor cystine knot (ICK)-gating modifier toxin that selectively inhibits Kv4 channels. To characterize the molecular determinants of interaction, we performed alanine scanning of the Kv4.3 S3b region. HpTx2-Kv4.3 interaction had an apparent K(d) value of 2.3 microM. Two alanine mutants in Kv4.3 increased K(d) values to 6.4 microM for V276A and 25 microM for L275A. Simultaneous mutation of both amino acids to alanine nearly eliminated toxin interaction. Unlike Hanatoxin and other well characterized ICK toxins, HpTx2 binding does not require a charged amino acid for interaction. To determine whether the identity of the S3b binding site amino acids altered HpTx2 specificity, we constructed Kv4.3 [LV275IF]. This mutation decreased the K(d) value to 0.54 microM, suggesting that the hydrophobic character of the putative binding site is the most important property for interaction with HpTx2. One mutant, N280A, caused stronger interaction of HpTx2 with Kv4.3; the K(d) value for Kv4.3 [N280A] was 0.26 microM. To understand Kv4.3-based transient outward currents in native tissues, we tested the affinity of HpTx2 for Kv4.3 coexpressed with KChIP2b. The toxin's K(d) value for Kv4.3 + KChIP2b was 0.95 microM. KChIP2b stabilizes the closed state of Kv4.3, suggesting that the increased toxin affinity is due to increased stabilization of the closed state. These data show that HpTx2 binding to Kv4.3 has aspects in common with other ICK gating modifier toxins but that the interventions that increase toxin affinity suggest flexibility toward channel binding that belies its unusual specificity for Kv4 channels.

Dihydropyridine block of voltage-dependent K+ currents in rat dorsal root ganglion neurons

The dihydropyridines nifedipine, nimodipine and Bay K 8644 are widely used as pharmacological tools to assess the contribution of L-type voltage-gated Ca(2+) channels to a variety of neuronal processes including synaptic transmission, excitability and second messenger signaling. These compounds are still used in neuronal preparations despite evidence from cardiac tissue and heterologous expression systems that they block several voltage-dependent K(+) (Kv) channels. Both because these compounds have been used to assess the relative contribution of L-type Ca(2+) channels to several different processes in dorsal root ganglion (DRG) neurons and because a relatively wide variety of Kv channels present in other neuronal populations is present in DRG neurons, we determined the extent to which dihydropyridines block Kv currents in these neurons. Standard whole cell patch clamp techniques were used to study acutely disassociated adult rat DRG neurons. All three dihydropyridines tested blocked Kv currents in DRG neurons; IC(50) values (concentration resulting in an inhibition that is 50% of maximum) for nifedipine and nimodipine-induced block of sustained Kv currents were 14.5 and 6.6 microM, respectively. The magnitude of sustained current block was 44+/-1.6%, 60+/-2%, and 56+/-2.9% with 10 microM nifedipine, nimodipine and Bay K 8644, respectively. Current block was occluded by neither 4-aminopyridine (5 mM) nor tetraethylammonium (135 mM). Dihydropyridine-induced block of Kv currents was not associated with a shift in the voltage-dependence of current activation or inactivation, the recovery from inactivation, or voltage dependent block. However, there was a small use-dependence to the dihydropyridine-induced block. Our results suggest that several types of Kv channels in DRG neurons are blocked by mechanisms distinct from those underlying block of Kv channels in cardiac myocytes. Importantly, our results suggest that if investigators wish to explore the contribution of L-type Ca(2+) channels to neuronal function, they should consider alternative strategies for the manipulation of these channels than the use of dihydropyridines.

Modification of K+ channel-drug interactions by ancillary subunits

Reconciling ion channel alpha-subunit expression with native ionic currents and their pharmacological sensitivity in target organs has proved difficult. In native tissue, many K(+) channel alpha-subunits co-assemble with ancillary subunits, which can profoundly affect physiological parameters including gating kinetics and pharmacological interactions. In this review, we examine the link between voltage-gated potassium ion channel pharmacology and the biophysics of ancillary subunits. We propose that ancillary subunits can modify the interaction between pore blockers and ion channels by three distinct mechanisms: changes in (1) binding site accessibility; (2) orientation of pore-lining residues; (3) the ability of the channel to undergo post-binding conformational changes. Each of these subunit-induced changes has implications for gating, drug affinity and use dependence of their respective channel complexes. A single subunit may modulate its associated alpha-subunit by more than one of these mechanisms. Voltage-gated potassium channels are the site of action of many therapeutic drugs. In addition, potassium channels interact with drugs whose primary target is another channel, e.g. the calcium channel blocker nifedipine, the sodium channel blocker quinidine, etc. Even when K(+) channel block is the intended mode of action, block of related channels in non-target organs, e.g. the heart, can result in major and potentially lethal side-effects. Understanding factors that determine specificity, use dependence and other properties of K(+) channel drug binding are therefore of vital clinical importance. Ancillary subunits play a key role in determining these properties in native tissue, and so understanding channel-subunit interactions is vital to understanding clinical pharmacology.

Cav1.2 calcium channels modulate the spiking pattern of hippocampal pyramidal cells

Ca(v)1.2 L-type calcium channels support hippocampal synaptic plasticity, likely by facilitating dendritic Ca2+ influx evoked by action potentials (AP) back-propagated from the soma. Ca2+ influx into hippocampal neurons during somatic APs is sufficient to activate signalling pathways associated with late phase LTP. Thus, mechanisms controlling AP firing of hippocampal neurons are of major functional relevance. We examined the excitability of CA1 pyramidal cells using somatic current-clamp recordings in brain slices from control type mice and mice with the Ca(v)1.2 gene inactivated in principal hippocampal neurons. Lack of the Ca(v)1.2 protein did not affect either affect basic characteristics, such as resting membrane potential and input resistance, or parameters of single action potentials (AP) induced by 5 ms depolarising current pulses. However, CA1 hippocampal neurons from control and mutant mice differed in their patterns of AP firing during 500 ms depolarising current pulses: threshold voltage for repetitive firing was shifted significantly by about 5 mV to more depolarised potentials in the mutant mice (p<0.01), and the latency until firing of the first AP was prolonged (73.2+/-6.6 ms versus 48.1+/- 7.8 ms in control; p<0.05). CA1 pyramidal cells from the mutant mice also showed a lowered initial spiking frequency within an AP train. In control cells, isradipine had matching effects, while BayK 8644 facilitated spiking. Our data demonstrate that Ca(v)1.2 channels are involved in regulating the intrinsic excitability of CA1 pyramidal neurons. This cellular mechanism may contribute to the known function of Ca(v)1.2 channels in supporting synaptic plasticity and memory.

Ancillary subunits and stimulation frequency determine the potency of chromanol 293B block of the KCNQ1 potassium channel

KCNQ1 (Kv7.1 or KvLQT1) encodes the alpha-subunit of a voltage-gated potassium channel found in tissues including heart, brain, epithelia and smooth muscle. Tissue-specific characteristics of KCNQ1 current are diverse, due to modification by ancillary subunits. In heart, KCNQ1 associates with KCNE1 (MinK), producing a slowly activating voltage-dependent channel. In epithelia, KCNQ1 co-assembles with KCNE3 (Mirp2) producing a constitutively open channel. Chromanol 293B is a selective KCNQ1 blocker. We studied drug binding and frequency dependence of 293B on KCNQ1 and ancillary subunits expressed in Xenopus oocytes. Ancillary subunits altered 293B potency up to 100-fold (IC(50) for KCNQ1 = 65.4 +/- 1.7 microm; KCNQ1/KCNE1 = 15.1 +/- 3.3 microm; KCNQ1/KCNE3 = 0.54 +/- 0.18 microm). Block of KCNQ1 and KCNQ1/KCNE3 was time independent, but 293B altered KCNQ1/KCNE1 activation. We therefore studied frequency-dependent block of KCNQ1/KCNE1. Repetitive rapid stimulation increased KCNQ1/KCNE1 current biphasically, and 293B abolished the slow component. KCNQ1/KCNE3[V72T] activates slowly with a KCNQ1/KCNE1-like phenotype, but retains the high affinity binding of KCNQ1/KCNE3, demonstrating that subunit-mediated changes in gating can be dissociated from subunit-mediated changes in affinity. This study demonstrates the KCNQ1 pharmacology is significantly altered by ancillary subunits. The response of KCNQ1 to specific blockers will therefore be critically dependent on the electrical stimulation pattern of the target organ. Furthermore, the dissociation between gating and overall affinity suggests that mutations in ancillary subunits can potentially strongly alter drug sensitivity without obvious functional changes in gating behaviour, giving rise to unexpected side-effects such as a predisposition to acquired long QT syndrome.

Interaction of riluzole with the closed inactivated state of Kv4.3 channels

The effect of riluzole on Kv4.3 was examined using the whole-cell patch-clamp technique. Riluzole inhibited the peak amplitude of Kv4.3 in a reversible, concentration-dependent manner with an IC(50) of 115.6 microM. Under control conditions, a good fit for the inactivation of Kv4.3 currents to a double exponential function, with the time constants of the fast component (tau(f)) and the slow component (tau(s)), was obtained. tau(f) was not altered by riluzole at concentrations up to 100 microM, but tau(s) became slower with increasing riluzole concentration, resulting in the crossover of the currents. The inhibition increased steeply with increasing channel activation at more positive potentials. In the full activation voltage range positive to (+)30 mV, however, no voltage-dependent inhibition was found. Riluzole shifted the voltage dependence of the steady-state inactivation of Kv4.3 in the hyperpolarizing direction in a concentration-dependent manner. However, the slope factor was not affected by riluzole. The K(i) for riluzole for interacting with the inactivated state of Kv4.3 was estimated from the concentration-dependent shift in the steady-state inactivation curve and was determined to be 1.2 muM. Under control conditions, closed state inactivation was fitted to a single exponential function. Riluzole caused a substantial acceleration in the closed state inactivation. In the presence of riluzole, the recovery from inactivation was slower than under control conditions. Riluzole induced a significant use-dependent inhibition of Kv4.3. These results suggest that riluzole inhibits Kv4.3 by binding to the closed inactivated state of the channels and that the unbinding of riluzole occurs from the closed state during depolarization.