Basolateral localisation of KCNQ1 potassium channels in MDCK cells: molecular identification of an N-terminal targeting motif

Clinically Relevant KCNQ1 Variants Causing KCNQ1-KCNE2 Gain-of-Function Affect the Ca2+ Sensitivity of the Channel

Dominant KCNQ1 variants are well-known for underlying cardiac arrhythmia syndromes. The two heterozygous KCNQ1 missense variants, R116L and P369L, cause an allelic disorder characterized by pituitary hormone deficiency and maternally inherited gingival fibromatosis. Increased K⁺ conductance upon co-expression of KCNQ1 mutant channels with the beta subunit KCNE2 is suggested to underlie the phenotype; however, the reason for KCNQ1-KCNE2 (Q1E2) channel gain-of-function is unknown. We aimed to discover the genetic defect in a single individual and three family members with gingival overgrowth and identified the KCNQ1 variants P369L and V185M, respectively. Patch-clamp experiments demonstrated increased constitutive K⁺ conductance of V185M-Q1E2 channels, confirming the pathogenicity of the novel variant. To gain insight into the pathomechanism, we examined all three disease-causing KCNQ1 mutants. Manipulation of the intracellular Ca²⁺ concentration prior to and during whole-cell recordings identified an impaired Ca²⁺ sensitivity of the mutant KCNQ1 channels. With low Ca²⁺, wild-type KCNQ1 currents were efficiently reduced and exhibited a pre-pulse-dependent cross-over of current traces and a high-voltage-activated component. These features were absent in mutant KCNQ1 channels and in wild-type channels co-expressed with calmodulin and exposed to high intracellular Ca²⁺. Moreover, co-expression of calmodulin with wild-type Q1E2 channels and loading the cells with high Ca²⁺ drastically increased Q1E2 current amplitudes, suggesting that KCNE2 normally limits the resting Q1E2 conductance by an increased demand for calcified calmodulin to achieve effective channel opening. Our data link impaired Ca²⁺ sensitivity of the KCNQ1 mutants R116L, V185M and P369L to Q1E2 gain-of-function that is associated with a particular KCNQ1 channelopathy.

Association Analysis of Candidate Gene Polymorphisms and Tinnitus in Young Musicians

INTRODUCTION

Subjective tinnitus, a perception of phantom sound, is a common otological condition that affects almost 15% of the general population. It is known that noise-induced hearing loss (NIHL) and tinnitus exhibit a high level of comorbidity in individuals exposed to intense noise and music. However, the influence of genetic variants associated with NIHL on tinnitus remains elusive. We hypothesized that young musicians carrying genetic variants associated with NIHL would exhibit a higher prevalence of tinnitus than their counterparts.

METHODS

To test this hypothesis, we analyzed the database by Bhatt et al. (2020) (originally developed by Phillips et al., 2015) that investigated the genetic links to NIHL in young college-aged musicians. The present study identified 186 participants (average age = 20.3 yrs, range = 18-25 yrs) with normal tympanometry and otoscopic findings and with no missing data. We included 19 single nucleotide polymorphisms in 13 cochlear genes that were previously associated with NIHL. The candidate genes include: KCNE1, KCNQ1, CDH23, GJB2, GJB4, KCNJ10, CAT, HSP70, PCDH70, MYH14, GRM7, PON2, and ESRRB.

RESULTS

We find that individuals with at least one minor allele of rs163171 (C > T) in KCNQ1 exhibit significantly higher odds of reporting tinnitus compared to individuals carrying the major allele of rs163171. KCNE1 rs2070358 revealed a suggestive association (p = 0.049) with tinnitus, but the FDR corrected p-value did not achieve statistical significance (p < 0.05). A history of ear infection and sound level tolerance showed a statistically significant association with tinnitus. Music exposure showed a suggestive association trend with tinnitus. Biological sex revealed a statistically significant association with distortion product otoacoustic emissions SNR measures.

CONCLUSIONS

We concluded that KCNQ1/KCNE1 voltage-gated potassium ion channel plays a critical role in the pathogenesis of NIHL and tinnitus. Further research is required to construct clinical tools for identifying genetically predisposed individuals well before they acquire NIHL and tinnitus.

Evidence for Effective Multiple K+-Current Inhibitions by Tolvaptan, a Non-peptide Antagonist of Vasopressin V2 Receptor

Tolvaptan (TLV), an oral non-peptide antagonist of vasopressin V₂ receptor, has been increasingly used for managements in patients with hyponatremia and/or syndrome of inappropriate antidiuretic hormone secretion. However, none of the studies have thus far been investigated with regard to its possible perturbations on membrane ion currents in endocrine or neuroendocrine cells. In our electrophysiological study, the whole-cell current recordings showed that the presence of TLV effectively and differentially suppressed the amplitude of delayed rectifier K⁺ (I_K(DR)) and M-type K⁺ current (I_K(M)) in pituitary GH₃ cells with an IC₅₀ value of 6.42 and 1.91 μM, respectively. This compound was also capable of shifting the steady-state activation curve of I_K(M) to less depolarized potential without any appreciable change in the gating charge of this current. TLV at a concentration greater than 10 μM also suppressed the amplitude of erg-mediated K⁺ current or the activity of large-conductance Ca²⁺-activated K⁺ channels; however, this compound failed to alter the amplitude of hyperpolarization-activated cation current in GH₃ cells. In vasopressin-preincubated GH₃ cells, TLV-mediated suppression of I_K(M) remained little altered. Under current-clamp condition, we also observed that addition of TLV increased the firing of spontaneous action potentials in GH₃ cells and further addition of flupirtine could reverse TLV-mediated elevation of the firing. In Madin-Darby canine kidney (MDCK) cells, the K⁺ current elicited by long ramp pulse was also effectively subject to inhibition by this compound. Findings from the present study were thus stated as saying that the suppression by TLV of multiple type K⁺ currents could be direct and independent of its antagonism of vasopressin V₂ receptors. Our study also reveals an important aspect that should be considered when assessing aquaretic effect of TLV or its structurally similar compounds.

Molecular cloning and functional expression of the K+ channel KV7.1 and the regulatory subunit KCNE1 from equine myocardium

BACKGROUND

The voltage-gated K⁺-channel K_V7.1 and the subunit KCNE1, encoded by the KCNQ1 and KCNE1 genes, respectively, are responsible for termination of the cardiac action potential. In humans, mutations in these genes can predispose patients to arrhythmias and sudden cardiac death (SCD).

AIM

To characterize equine K_V7.1/KCNE1 currents and compare them to human K_V7.1/KCNE1 currents to determine whether K_V7.1/KCNE1 plays a similar role in equine and human hearts.

METHODS

mRNA encoding K_V7.1 and KCNE1 was isolated from equine hearts, sequenced, and cloned into expression vectors. The channel subunits were heterologously expressed in Xenopus laevis oocytes or CHO-K1 cells and characterized using voltage-clamp techniques.

RESULTS

Equine K_V7.1/KCNE1 expressed in CHO-K1 cells exhibited electrophysiological properties that are overall similar to the human orthologs; however, a slower deactivation was found which could result in more open channels at fast rates.

CONCLUSION

The results suggest that the equine K_V7.1/KCNE1 channel may be important for cardiac repolarization and this could indicate that horses are susceptible to SCD caused by mutations in KCNQ1 and KCNE1.

Screen-based identification and validation of four new ion channels as regulators of renal ciliogenesis

To investigate the contribution of ion channels to ciliogenesis, we carried out a small interfering RNA (siRNA)-based reverse genetics screen of all ion channels in the mouse genome in murine inner medullary collecting duct kidney cells. This screen revealed four candidate ion channel genes: Kcnq1, Kcnj10, Kcnf1 and Clcn4. We show that these four ion channels localize to renal tubules, specifically to the base of primary cilia. We report that human KCNQ1 Long QT syndrome disease alleles regulate renal ciliogenesis; KCNQ1-p.R518X, -p.A178T and -p.K362R could not rescue ciliogenesis after Kcnq1-siRNA-mediated depletion in contrast to wild-type KCNQ1 and benign KCNQ1-p.R518Q, suggesting that the ion channel function of KCNQ1 regulates ciliogenesis. In contrast, we demonstrate that the ion channel function of KCNJ10 is independent of its effect on ciliogenesis. Our data suggest that these four ion channels regulate renal ciliogenesis through the periciliary diffusion barrier or the ciliary pocket, with potential implication as genetic contributors to ciliopathy pathophysiology. The new functional roles of a subset of ion channels provide new insights into the disease pathogenesis of channelopathies, which might suggest future therapeutic approaches.

Activation of m1 muscarinic acetylcholine receptor induces surface transport of KCNQ channels through a CRMP-2-mediated pathway

Neuronal excitability is strictly regulated by various mechanisms, including modulation of ion channel activity and trafficking. Stimulation of m1 muscarinic acetylcholine receptor (also known as CHRM1) increases neuronal excitability by suppressing the M-current generated by the Kv7/KCNQ channel family. We found that m1 muscarinic acetylcholine receptor stimulation also triggers surface transport of KCNQ subunits. This receptor-induced surface transport was observed with KCNQ2 as well as KCNQ3 homomeric channels, but not with Kv3.1 channels. Deletion analyses identified that a conserved domain in a proximal region of the N-terminal tail of KCNQ protein is crucial for this surface transport--the translocation domain. Proteins that bind to this domain were identified as α- and β-tubulin and collapsin response mediator protein 2 (CRMP-2; also known as DPYSL2). An inhibitor of casein kinase 2 (CK2) reduced tubulin binding to the translocation domain, whereas an inhibitor of glycogen synthase kinase 3 (GSK3) facilitated CRMP-2 binding to the translocation domain. Consistently, treatment with the GSK3 inhibitor enhanced receptor-induced KCNQ2 surface transport. M-current recordings from neurons showed that treatment with a GSK3 inhibitor shortened the duration of muscarinic suppression and led to over-recovery of the M-current. These results suggest that m1 muscarinic acetylcholine receptor stimulates surface transport of KCNQ channels through a CRMP-2-mediated pathway.

Protein kinase A stimulates Kv7.1 surface expression by regulating Nedd4-2-dependent endocytic trafficking

The potassium channel Kv7.1 plays critical physiological roles in both heart and epithelial tissues. In heart, Kv7.1 and the accessory subunit KCNE1 forms the slowly activating delayed-rectifier potassium current current, which is enhanced by protein kinase A (PKA)-mediated phosphorylation. The observed current increase requires both phosphorylation of Kv7.1 and the presence of KCNE1. However, PKA also stimulates Kv7.1 currents in epithelial tissues, such as colon, where the channel does not coassemble with KCNE1. Here, we demonstrate that PKA activity significantly impacts the subcellular localization of Kv7.1 in Madin-Darby canine kidney cells. While PKA inhibition reduced the fraction of channels at the cell surface, PKA activation increased it. We show that PKA inhibition led to intracellular accumulation of Kv7.1 in late endosomes/lysosomes. By mass spectroscopy we identified eight phosphorylated residues on Kv7.1, however, none appeared to play a role in the observed response. Instead, we found that PKA acted by regulating endocytic trafficking involving the ubiquitin ligase Nedd4-2. We show that a Nedd4-2-resistant Kv7.1-mutant displayed significantly reduced intracellular accumulation upon PKA inhibition. Similar effects were observed upon siRNA knockdown of Nedd4-2. However, although Nedd4-2 is known to regulate Kv7.1 by ubiquitylation, biochemical analyses demonstrated that PKA did not influence the amount of Nedd4-2 bound to Kv7.1 or the ubiquitylation level of the channel. This suggests that PKA influences Nedd4-2-dependent Kv7.1 transport though a different molecular mechanism. In summary, we identify a novel mechanism whereby PKA can increase Kv7.1 current levels, namely by regulating Nedd4-2-dependent Kv7.1 transport.

High incidence of functional ion-channel abnormalities in a consecutive Long QT cohort with novel missense genetic variants of unknown significance

The Long QT syndrome (LQTS) is a disorder characterized by a prolongation of the QT interval and a propensity to ventricular tachyarrhythmias, which may lead to syncope, cardiac arrest, or sudden death. Our objective was to (1) determine the incidence of variants with unknown significance (VUS) in a cohort of consecutive LQTS patients and (2) to determine the percentage of those with novel missense VUS that have demonstrable functional channel abnormalities from a single referral center. We performed genetic screening of candidate genes in 39 probands with a diagnosis of LQTS to identify mutations and variants. Seven variants of unknown significance were identified, six were missense variants and one was a splice site variant. We investigated the six novel missense VUS in five patients; three missense variants in KCNQ1 (L236R, W379R, Y522S) and three missense variants in KCNH2 (R35W, S620G, V491I). We employed two-electrode voltage-clamp experiments in Xenopus laevis oocytes and confocal imaging to characterize the novel missense mutations functionally. We revealed electrophysiological and trafficking loss-of-function phenotypes. This report emphasizes the frequency of adverse channel function in patients with LQTS and the importance of heterologous studies to define channel function.

Cardiac potassium channel subtypes: new roles in repolarization and arrhythmia

About 10 distinct potassium channels in the heart are involved in shaping the action potential. Some of the K+ channels are primarily responsible for early repolarization, whereas others drive late repolarization and still others are open throughout the cardiac cycle. Three main K+ channels drive the late repolarization of the ventricle with some redundancy, and in atria this repolarization reserve is supplemented by the fairly atrial-specific KV1.5, Kir3, KCa, and K2P channels. The role of the latter two subtypes in atria is currently being clarified, and several findings indicate that they could constitute targets for new pharmacological treatment of atrial fibrillation. The interplay between the different K+ channel subtypes in both atria and ventricle is dynamic, and a significant up- and downregulation occurs in disease states such as atrial fibrillation or heart failure. The underlying posttranscriptional and posttranslational remodeling of the individual K+ channels changes their activity and significance relative to each other, and they must be viewed together to understand their role in keeping a stable heart rhythm, also under menacing conditions like attacks of reentry arrhythmia.

Biology of the KCNQ1 Potassium Channel

Ion channels are essential for basic cellular function and for processes including sensory perception and intercellular communication in multicellular organisms. Voltage-gated potassium (Kv) channels facilitate dynamic cellular repolarization during an action potential, opening in response to membrane depolarization to facilitate K+ efflux. In both excitable and nonexcitable cells other, constitutively active, K+ channels provide a relatively constant repolarizing force to control membrane potential, ion homeostasis, and secretory processes. Of the forty known human Kv channel pore-forming α subunits that coassemble in various combinations to form the fundamental tetrameric channel pore and voltage sensor module, KCNQ1 is unique. KCNQ1 stands alone in having the capacity to form either channels that are voltage-dependent and require membrane depolarization for activation, or constitutively active channels. In mammals, KCNQ1 regulates processes including gastric acid secretion, thyroid hormone biosynthesis, salt and glucose homeostasis, and cell volume and in some species is required for rhythmic beating of the heart. In this review, the author discusses the unique functional properties, regulation, cell biology, diverse physiological roles, and involvement in human disease states of this chameleonic K+ channel.

A phosphoinositide 3-kinase (PI3K)-serum- and glucocorticoid-inducible kinase 1 (SGK1) pathway promotes Kv7.1 channel surface expression by inhibiting Nedd4-2 protein

Epithelial cell polarization involves several kinase signaling cascades that eventually divide the surface membrane into an apical and a basolateral part. One kinase, which is activated during the polarization process, is phosphoinositide 3-kinase (PI3K). In MDCK cells, the basolateral potassium channel Kv7.1 requires PI3K activity for surface-expression during the polarization process. Here, we demonstrate that Kv7.1 surface expression requires tonic PI3K activity as PI3K inhibition triggers endocytosis of these channels in polarized MDCK. Pharmacological inhibition of SGK1 gave similar results as PI3K inhibition, whereas overexpression of constitutively active SGK1 overruled it, suggesting that SGK1 is the primary downstream target of PI3K in this process. Furthermore, knockdown of the ubiquitin ligase Nedd4-2 overruled PI3K inhibition, whereas a Nedd4-2 interaction-deficient Kv7.1 mutant was resistant to both PI3K and SGK1 inhibition. Altogether, these data suggest that a PI3K-SGK1 pathway stabilizes Kv7.1 surface expression by inhibiting Nedd4-2-dependent endocytosis and thereby demonstrates that Nedd4-2 is a key regulator of Kv7.1 localization and turnover in epithelial cells.

Trafficking of the IKs -complex in MDCK cells: site of subunit assembly and determinants of polarized localization

The voltage-gated potassium channel KV 7.1 is regulated by non-pore forming regulatory KCNE β-subunits. Together with KCNE1, it forms the slowly activating delayed rectifier potassium current IKs . However, where the subunits assemble and which of the subunits determines localization of the IKs -complex has not been unequivocally resolved yet. We employed trafficking-deficient KV 7.1 and KCNE1 mutants to investigate IKs trafficking using the polarized Madin-Darby Canine Kidney cell line. We find that the assembly happens early in the secretory pathway but provide three lines of evidence that it takes place in a post-endoplasmic reticulum compartment. We demonstrate that KV 7.1 targets the IKs -complex to the basolateral membrane, but that KCNE1 can redirect the complex to the apical membrane upon mutation of critical KV 7.1 basolateral targeting signals. Our data provide a possible explanation to the fact that KV 7.1 can be localized apically or basolaterally in different epithelial tissues and offer a solution to divergent literature results regarding the effect of KCNE subunits on the subcellular localization of KV 7.1/KCNE complexes.

Independent trafficking of the KCNQ1 K+ channel and H+-K+-ATPase in gastric parietal cells from mice

Gastric acid secretion by the H(+)-K(+)-ATPase at the apical surface of activated parietal cells requires luminal K(+) provided by the KCNQ1/KCNE2 K(+) channel. However, little is known about the trafficking and relative spatial distribution of KCNQ1 and H(+)-K(+)-ATPase in resting and activated parietal cells and the capacity of KCNQ1 to control acid secretion. Here we show that inhibition of KCNQ1 activity quickly curtails gastric acid secretion in vivo, even when the H(+)-K(+)-ATPase is permanently anchored in the apical membrane, demonstrating a key role of the K(+) channel in controlling acid secretion. Three-dimensional imaging analysis of isolated mouse gastric units revealed that the majority of KCNQ1 resides in an intracytoplasmic, Rab11-positive compartment in resting parietal cells, distinct from H(+)-K(+)-ATPase-enriched tubulovesicles. Upon activation, there was a significant redistribution of H(+)-K(+)-ATPase and KCNQ1 from intracytoplasmic compartments to the apical secretory canaliculi. Significantly, high Förster resonance energy transfer was detected between H(+)-K(+)-ATPase and KCNQ1 in activated, but not resting, parietal cells. These findings demonstrate that H(+)-K(+)-ATPase and KCNQ1 reside in independent intracytoplasmic membrane compartments, or membrane domains, and upon activation of parietal cells, both membrane proteins are transported, possibly via Rab11-positive recycling endosomes, to apical membranes, where the two molecules are closely physically opposed. In addition, these studies indicate that acid secretion is regulated by independent trafficking of KCNQ1 and H(+)-K(+)-ATPase.

L-364,373 (R-L3) enantiomers have opposite modulating effects on IKs in mammalian ventricular myocytes

Activators of the slow delayed rectifier K⁺ current (IKs) have been suggested as promising tools for suppressing ventricular arrhythmias due to prolongation of repolarization. Recently, L-364,373 (R-L3) was nominated to activate IKs in myocytes from several species; however, in some studies, it failed to activate IKs. One later study suggested opposite modulating effects from the R-L3 enantiomers as a possible explanation for this discrepancy. Therefore, we analyzed the effect of the RL-3 enantiomers on IKs in ventricular mammalian myocytes, by applying standard microelectrode and whole-cell patch-clamp techniques at 37 °C. We synthesized 2 substances, ZS_1270B (right) and ZS_1271B (left), the 2 enantiomers of R-L3. In rabbit myocytes, ZS_1270B enhanced the IKs tail current by approximately 30%, whereas ZS_1271B reduced IKs tails by 45%. In guinea pig right ventricular preparations, ZS_1270B shortened APD90 (action potential duration measured at 90% repolarization) by 12%, whereas ZS_1271B lengthened it by approximately 15%. We concluded that R-L3 enantiomers in the same concentration range indeed have opposite modulating effects on IKs, which may explain why the racemic drug R-L3 previously failed to activate IKs. ZS_1270B is a potent IKs activator, therefore, this substance is appropriate to test whether IKs activators are ideal tools to suppress ventricular arrhythmias originating from prolongation of action potentials.

Founder mutations characterise the mutation panorama in 200 Swedish index cases referred for Long QT syndrome genetic testing

Background

Long QT syndrome (LQTS) is an inherited arrhythmic disorder characterised by prolongation of the QT interval on ECG, presence of syncope and sudden death. The symptoms in LQTS patients are highly variable, and genotype influences the clinical course. This study aims to report the spectrum of LQTS mutations in a Swedish cohort.

Methods

Between March 2006 and October 2009, two hundred, unrelated index cases were referred to the Department of Clinical Genetics, Umeå University Hospital, Sweden, for LQTS genetic testing. We scanned five of the LQTS-susceptibility genes (KCNQ1, KCNH2, SCN5A, KCNE1, and KCNE2) for mutations by DHPLC and/or sequencing. We applied MLPA to detect large deletions or duplications in the KCNQ1, KCNH2, SCN5A, KCNE1, and KCNE2 genes. Furthermore, the gene RYR2 was screened in 36 selected LQTS genotype-negative patients to detect cases with the clinically overlapping disease catecholaminergic polymorphic ventricular tachycardia (CPVT).

Results

In total, a disease-causing mutation was identified in 103 of the 200 (52%) index cases. Of these, altered exon copy numbers in the KCNH2 gene accounted for 2% of the mutations, whereas a RYR2 mutation accounted for 3% of the mutations. The genotype-positive cases stemmed from 64 distinct mutations, of which 28% were novel to this cohort. The majority of the distinct mutations were found in a single case (80%), whereas 20% of the mutations were observed more than once. Two founder mutations, KCNQ1 p.Y111C and KCNQ1 p.R518*, accounted for 25% of the genotype-positive index cases. Genetic cascade screening of 481 relatives to the 103 index cases with an identified mutation revealed 41% mutation carriers who were at risk of cardiac events such as syncope or sudden unexpected death.

Conclusion

In this cohort of Swedish index cases with suspected LQTS, a disease-causing mutation was identified in 52% of the referred patients. Copy number variations explained 2% of the mutations and 3 of 36 selected cases (8%) harboured a mutation in the RYR2 gene. The mutation panorama is characterised by founder mutations (25%), even so, this cohort increases the amount of known LQTS-associated mutations, as approximately one-third (28%) of the detected mutations were unique.

Regions of KCNQ K(+) channels controlling functional expression

KCNQ1–5 α-subunits assemble to form K⁺ channels that play critical roles in the function of numerous tissues. The channels are tetramers of subunits containing six transmembrane domains. Each subunit consists of a pore region (S5-pore-S6) and a voltage-sensor domain (S1-S4). Despite similar structures, KCNQ2 and KCNQ3 homomers yield small current amplitudes compared to other KCNQ homomers and KCNQ2/3 heteromers. Two major mechanisms have been suggested as governing functional expression. The first involves control of channel trafficking to the plasma membrane by the distal part of the C-terminus, containing two coiled–coiled domains, required for channel trafficking and assembly. The proximal half of the C-terminus is the crucial region for channel modulation by signaling molecules such as calmodulin (CaM), which may mediate C- and N-terminal interactions. The N-terminus of KCNQ channels has also been postulated as critical for channel surface expression. The second mechanism suggests networks of interactions between the pore helix and the selectivity filter (SF), and between the pore helix and the S6 domain that govern KCNQ current amplitudes. Here, we summarize the role of these different regions in expression of functional KCNQ channels.

Cardiac channelopathies and sudden infant death syndrome

Sudden infant death syndrome (SIDS) is always a devastating and unexpected occurrence. SIDS is the leading cause of death in the first 6 months after birth in the industrialized world. Since the discovery in 1998 of long QT syndrome as an underlying substrate for SIDS, around 10-20% of SIDS cases have been proposed as being caused by genetic variants in either ion channel or ion channel-associated proteins. Until now, 10 cardiac channelopathy susceptibility genes have been found to be implicated in the pathogenesis of SIDS. Four of the genes encode cardiac ion channel α-subunits, 3 genes encode ion channel β-subunits, and 3 genes encode other channel-interacting proteins. All 10 genes have been associated with primary electrical heart diseases. SIDS may hereby be the initial symptom of rare primary electric channelopathies such as long QT, short QT and Brugada syndrome, as well as catecholaminergic polymorphic ventricular tachycardia. In this review we describe the functional role of sodium, potassium and calcium channels in propagation, depolarization and repolarization in the context of the 4 arrhythmogenic diseases reported to be associated with SIDS. Lastly, the possibility of postmortem genetic testing and potential recommendations on how to deal with family members are discussed.

Kv7.1 surface expression is regulated by epithelial cell polarization

The potassium channel KV7.1 is expressed in the heart where it contributes to the repolarization of the cardiac action potential. In addition, KV7.1 is expressed in epithelial tissues where it plays a role in salt and water transport. Mutations in the kcnq1 gene can lead to long QT syndrome and deafness, and several mutations have been described as trafficking mutations. To learn more about the basic mechanisms that regulate KV7.1 surface expression, we have investigated the trafficking of KV7.1 during the polarization process of the epithelial cell line Madin-Darby Canine Kidney (MDCK) using a modified version of the classical calcium switch. We discovered that KV7.1 exhibits a very dynamic localization pattern during the calcium switch. When MDCK cells are kept in low calcium medium, KV7.1 is mainly observed at the plasma membrane. During the first hours of the switch, KV7.1 is removed from the plasma membrane and an intracellular accumulation in the endoplasmic reticulum (ER) is observed. The channel is retained in the ER until the establishment of the lateral membranes at which point KV7.1 is released from the ER and moves to the plasma membrane. Our data furthermore suggest that while the removal of KV7.1 from the cell surface and its accumulation in the ER could involve activation of protein kinase C, the subsequent release of KV7.1 from the ER depends on phosphoinositide 3-kinase (PI3K) activation. In conclusion, our results demonstrate that KV7.1 surface expression is regulated by signaling mechanisms involved in epithelial cell polarization in particular signaling cascades involving protein kinase C and PI3K.

Genetic dissection reveals unexpected influence of beta subunits on KCNQ1 K+ channel polarized trafficking in vivo

Targeted deletion of the Kcne2 potassium channel β subunit gene ablates gastric acid secretion and predisposes to gastric neoplasia in mice. Here, we discovered that Kcne2 deletion basolaterally reroutes the Kcnq1 α subunit in vivo in parietal cells (PCs), in which the normally apical location of the Kcnq1-Kcne2 channel facilitates its essential role in gastric acid secretion. Quantitative RT-PCR and Western blotting revealed that Kcne2 deletion remodeled fundic Kcne3 (2.9±0.8-fold mRNA increase, n=10; 5.3±0.4-fold protein increase, n=7) but not Kcne1, 4, or 5, and resulted in basolateral Kcnq1-Kcne3 complex formation in Kcne2(-/-) PCs. Concomitant targeted deletion of Kcne3 (creating Kcne2(-/-)Kcne3(-/-) mice) restored PC apical Kcnq1 localization without Kcne1, 4, or 5 remodeling (assessed by quantitative RT-PCR; n=5-10), indicating Kcne3 actively, basolaterally rerouted Kcnq1 in Kcne2(-/-) PCs. Despite this, Kcne3 deletion exacerbated gastric hyperplasia in Kcne2(-/-) mice, and both hypochlorhydria and hyperplasia in Kcne2(+/-) mice, suggesting that Kcne3 up-regulation was beneficial in Kcne2-depleted PCs. The findings reveal, in vivo, Kcne-dependent α subunit polarized trafficking and the existence and consequences of potassium channel β subunit remodeling.

Tyrosine residues in the cytoplasmic domains affect sorting and fusion activity of the Nipah virus glycoproteins in polarized epithelial cells

The highly pathogenic Nipah virus (NiV) is aerially transmitted and causes a systemic infection after entering the respiratory tract. Airway epithelia are thus important targets in primary infection. Furthermore, virus replication in the mucosal surfaces of the respiratory or urinary tract in later phases of infection is essential for virus shedding and transmission. So far, the mechanisms of NiV replication in epithelial cells are poorly elucidated. In the present study, we provide evidence that bipolar targeting of the two NiV surface glycoproteins G and F is of biological importance for fusion in polarized epithelia. We demonstrate that infection of polarized cells induces focus formation, with both glycoproteins located at lateral membranes of infected cells adjacent to uninfected cells. Supporting the idea of a direct spread of infection via lateral cell-to-cell fusion, we could identify basolateral targeting signals in the cytoplasmic domains of both NiV glycoproteins. Tyrosine 525 in the F protein is part of an endocytosis signal and is also responsible for basolateral sorting. Surprisingly, we identified a dityrosine motif at position 28/29 in the G protein, which mediates polarized targeting. A dileucine motif predicted to function as sorting signal is not involved. Mutation of the targeting signal in one of the NiV glycoproteins prevented the fusion of polarized cells, suggesting that basolateral or bipolar F and G expression facilitates the spread of NiV within epithelial cell monolayers, thereby contributing to efficient virus spread in mucosal surfaces in early and late phases of infection.

Identification of a Kir3.4 mutation in congenital long QT syndrome

Congenital long QT syndrome (LQTS) is a hereditary disorder that leads to sudden cardiac death secondary to fatal cardiac arrhythmias. Although many genes for LQTS have been described, the etiology remains unknown in 30%-40% of cases. In the present study, a large Chinese family (four generations, 49 individuals) with autosomal-dominant LQTS was clinically evaluated. Genome-wide linkage analysis was performed by using polymorphic microsatellite markers to map the genetic locus, and positional candidate genes were screened by sequencing for mutations. The expression pattern and functional characteristics of the mutated protein were investigated by western blotting and patch-clamp electrophysiology. The genetic locus of the LQTS-associated gene was mapped to chromosome 11q23.3-24.3. A heterozygous mutation (Kir3.4-Gly387Arg) was identified in the G protein-coupled, inwardly rectifying potassium channel subunit Kir3.4, encoded by the KCNJ5 gene. The Kir3.4-Gly387Arg mutation was present in all nine affected family members and absent in 528 ethnically matched controls. Western blotting of human cardiac tissue demonstrated significant Kir3.4 expression levels in the cardiac ventricles. Heterologous expression studies with Kir3.4-Gly387Arg revealed a loss-of-function electrophysiological phenotype resulting from reduced plasma membrane expression. Our findings suggest a role for Kir3.4 in the etiology of LQTS.

KV7 channelopathies

KV7 voltage-gated potassium channels, encoded by the KCNQ gene family, have caught increasing interest of the scientific community for their important physiological roles, which are emphasized by the fact that four of the five so far identified members are related to different hereditary diseases. Furthermore, these channels prove to be attractive pharmacological targets for treating diseases characterized by membrane hyperexcitability. KV7 channels are expressed in brain, heart, thyroid gland, pancreas, inner ear, muscle, stomach, and intestines. They give rise to functionally important potassium currents, reduction of which results in pathologies such as long QT syndrome, diabetes, neonatal epilepsy, neuromyotonia, or progressive deafness. Here, we summarize some key traits of KV7 channels and review how their molecular deficiencies could explain diverse disease phenotypes. We also assess the therapeutic potential of KV7 channels; in particular, how the activation of KV7 channels by the compounds retigabine and R-L3 may be useful for treatment of epilepsy or cardiac arrhythmia.

Differential expression of KCNQ1 K+ channel in tubular cells of frog kidney

The aim of this study was to evaluate KCNQ1 K⁺ channel expression in the frog kidney of Rana esculenta. KCNQ1 K⁺ channel, also known as KvLQT1, is the pore forming α-subunit of the I_Ks K⁺ channel, a delayed rectifier voltage-gated K⁺ channel, which has an important role in water and salt transport in the kidney and gastrointestinal tract. The expression of KCNQ1 K⁺ channel along tubular epithelium differs from species to species. In the present study the expression of KCNQ1 K⁺ channel in the frog kidney has been demonstrated by immunohistochemistry. The presence of KCNQ1 K⁺ channel was demonstrated in the epithelial cells of distal convoluted tubule and collecting duct. However, the pattern of expression of KCNQ1 K⁺ channel differs between distal convoluted tubules and collecting duct. All epithelial cells of distal convoluted tubules revealed basolateral expression of KCNQ1 K⁺ channel. On the contrary, only the single cells of collecting duct, probably intercalated cells, showed diffuse cell surface staining with antibodies against KCNQ1 K⁺ channel. These findings suggest that KCNQ1 K⁺ channel has cell-specific roles in renal potassium ion transport.

Repolarization of the cardiac action potential. Does an increase in repolarization capacity constitute a new anti-arrhythmic principle?

The cardiac action potential can be divided into five distinct phases designated phases 0-4. The exact shape of the action potential comes about primarily as an orchestrated function of ion channels. The present review will give an overview of ion channels involved in generating the cardiac action potential with special emphasis on potassium channels involved in phase 3 repolarization. In humans, these channels are primarily K(v)11.1 (hERG1), K(v)7.1 (KCNQ1) and K(ir)2.1 (KCNJ2) being the responsible alpha-subunits for conducting I(Kr), I(Ks) and I(K1). An account will be given about molecular components, biophysical properties, regulation, interaction with other proteins and involvement in diseases. Both loss and gain of function of these currents are associated with different arrhythmogenic diseases. The second part of this review will therefore elucidate arrhythmias and subsequently focus on newly developed chemical entities having the ability to increase the activity of I(Kr), I(Ks) and I(K1). An evaluation will be given addressing the possibility that this novel class of compounds have the ability to constitute a new anti-arrhythmic principle. Experimental evidence from in vitro, ex vivo and in vivo settings will be included. Furthermore, conceptual differences between the short QT syndrome and I(Kr) activation will be accounted for.

Low Incidence of Sudden Cardiac Death in a Swedish Y111C Type 1 Long-QT syndrome Population

Background— A 10% cumulative incidence and a 0.3% per year incidence rate of sudden cardiac death in patients younger than 40 years and without therapy have been reported in type 1 long-QT syndrome. The Y111C-KCNQ1 mutation causes a severe phenotype in vitro, suggesting a high-risk mutation. This study investigated the phenotype among Y111C-KCNQ1 mutation carriers in the Swedish population with a focus on life-threatening cardiac events.

Methods and Results— We identified 80 mutation carriers in 15 index families, segregating the Y111C-KCNQ1 mutation during a national inventory of mutations causing the long-QT syndrome. Twenty-four mutation carriers <40 years experienced syncope (30%). One mutation carrier had an aborted cardiac arrest (1.25%). No case of sudden cardiac death was reported during a mean nonmedicated follow-up of 25�20 years. This corresponds to a low incidence rate of life-threatening cardiac events (0.05%/year versus 0.3%/year, P =0.025). In 8 Y111C families connected by a common ancestor, the natural history of the mutation was assessed by investigating the survival over the age of 40 years for 107 nonmedicated ascertained mutation carriers (n=24) and family members (n=83) born between 1873 and 1968. In total, 4 deaths in individuals younger than 40 years were noted: 1 case of noncardiac death and 3 infant deaths between 1873 and 1915.

Conclusions— The dominant-negative Y111C-KCNQ1 mutation, associated with a severe phenotype in vitro, presents with a low incidence of life-threatening cardiac events in a Swedish population. This finding of discrepancy emphasizes the importance of clinical observations in the risk stratification of long-QT syndrome.

MinK-dependent internalization of the IKs potassium channel

AIMS

KCNQ1-MinK potassium channel complexes (4alpha:2beta stoichiometry) generate IKs, the slowly activating human cardiac ventricular repolarization current. The MinK ancillary subunit slows KCNQ1 activation, eliminates its inactivation, and increases its unitary conductance. However, KCNQ1 transcripts outnumber MinK transcripts five to one in human ventricles, suggesting KCNQ1 also forms other heteromeric or even homomeric channels there. Mechanisms governing which channel types prevail have not previously been reported, despite their significance: normal cardiac rhythm requires tight control of IKs density and kinetics, and inherited mutations in KCNQ1 and MinK can cause ventricular fibrillation and sudden death. Here, we describe a novel mechanism for this control.

METHODS AND RESULTS

Whole-cell patch-clamping, confocal immunofluorescence microscopy, antibody feeding, biotin feeding, fluorescent transferrin feeding, and protein biochemistry techniques were applied to COS-7 cells heterologously expressing KCNQ1 with wild-type or mutant MinK and dynamin 2 and to native IKs channels in guinea-pig myocytes. KCNQ1-MinK complexes, but not homomeric KCNQ1 channels, were found to undergo clathrin- and dynamin 2-dependent internalization (DDI). Three sites on the MinK intracellular C-terminus were, in concert, necessary and sufficient for DDI. Gating kinetics and sensitivity to XE991 indicated that DDI decreased cell-surface KCNQ1-MinK channels relative to homomeric KCNQ1, decreasing whole-cell current but increasing net activation rate; inhibiting DDI did the reverse.

CONCLUSION

The data redefine MinK as an endocytic chaperone for KCNQ1 and present a dynamic mechanism for controlling net surface Kv channel subunit composition-and thus current density and gating kinetics-that may also apply to other alpha-beta type Kv channel complexes.

Long QT syndrome-associated mutations in KCNQ1 and KCNE1 subunits disrupt normal endosomal recycling of IKs channels

Physical and emotional stress is accompanied by release of stress hormones such as the glucocorticoid cortisol. This hormone upregulates the serum- and glucocorticoid-inducible kinase (SGK)1, which in turn stimulates I(Ks), a slow delayed rectifier potassium current that mediates cardiac action potential repolarization. Mutations in I(Ks) channel alpha (KCNQ1, KvLQT1, Kv7.1) or beta (KCNE1, IsK, minK) subunits cause long QT syndrome (LQTS), an inherited cardiac arrhythmia associated with increased risk of sudden death. Together with the GTPases RAB5 and RAB11, SGK1 facilitates membrane recycling of KCNQ1 channels. Here, we show altered SGK1-dependent regulation of LQTS-associated mutant I(Ks) channels. Whereas some mutant KCNQ1 channels had reduced basal activity but were still activated by SGK1, currents mediated by KCNQ1(Y111C) or KCNQ1(L114P) were paradoxically reduced by SGK1. Heteromeric channels coassembled of wild-type KCNQ1 and the LQTS-associated KCNE1(D76N) mutant were similarly downregulated by SGK1 because of a disrupted RAB11-dependent recycling. Mutagenesis experiments indicate that stimulation of I(Ks) channels by SGK1 depends on residues H73, N75, D76, and P77 in KCNE1. Identification of the I(Ks) recycling pathway and its modulation by stress-stimulated SGK1 provides novel mechanistic insight into potentially fatal cardiac arrhythmias triggered by physical or psychological stress.

KCNQ-encoded channels regulate Na+ transport across H441 lung epithelial cells

H441 cells are a model of absorptive airway epithelia that are characterised by a pronounced apical Na+ flux through amiloride-sensitive Na+ channels. The flux of Na+ is intimately linked to Na+ handling by the cell as well as the membrane potential across the apical membrane. As KCNQ-encoded K+ channels influence chloride secretion in gastrointestinal epithelia, the goal of the present study was to ascertain the expression of KCNQ genes in H441 cells and determine the functional role of the expression products. Message for KCNQ3 and KCNQ5 was detected by RT-polymerase chain reaction and the translated proteins were observed by immunocytochemistry. Ussing experiments showed that the pan-KCNQ channel blocker XE991, but not KCNQ1 selective blockers, reduced the short circuit current and the amiloride-sensitive component. These data show for the first time that potassium channels encoded by KCNQ3 or KCNQ5 are crucial determinants of epithelial Na+ flux.

KCNQ1 and KCNE1 K+ channel components are involved in early left-right patterning in Xenopus laevis embryos

Several ion transporters have been implicated in left-right (LR) patterning. Here, we characterize a new component of the early bioelectrical circuit: the potassium channel KCNQ1 and its accessory subunit KCNE1. Having cloned the native Xenopus versions of both genes, we show that both are asymmetrically localized as maternal proteins during the first few cleavages of frog embryo development in a process dependent on microtubule and actin organization. Molecular loss-of-function using dominant negative constructs demonstrates that both gene products are required for normal LR asymmetry. We propose a model whereby these channels provide an exit path for K(+) ions brought in by the H(+),K(+)-ATPase. This physiological module thus allows the obligate but electroneutral H(+),K(+)-ATPase to generate an asymmetric voltage gradient on the left and right sides. Our data reveal a new, bioelectrical component of the mechanisms patterning a large-scale axis in vertebrate embryogenesis.

Cellular distribution of the potassium channel KCNQ1 in normal mouse kidney

Mechanisms of K(+) secretion and absorption along the collecting duct are not understood fully. Because KCNQ1 participates in K(+) secretion within the inner ear and stomach, distribution of KCNQ1 in mouse kidney was studied using Northern and Western analyses, RT-PCR of isolated tubules, and immunohistochemistry. Northern blots demonstrated KCNQ1 transcripts in whole kidney. RT-PCR showed KCNQ1 mRNA in isolated distal convoluted tubule (DCT), connecting segment (CNT), collecting ducts (CD), and glomeruli. Immunoblots of kidney and stomach revealed a approximately 75-kDa protein, the expected mobility for KCNQ1. KCNQ1 was detected by immunohistochemistry throughout the distal nephron and CD. Thick ascending limbs exhibited weak basolateral immunolabel. In DCT and CNT cells, immunolabel was intense and basolateral, although KCNQ1 label was stronger in late than in early DCT. Initial collecting tubule and cortical CD KCNQ1 immunolabel was predominantly diffuse, but many cells exhibited discrete apical label. Double-labeling experiments demonstrated that principal cells, type B intercalated cells, and a few type A intercalated cells exhibited distinct apical KCNQ1 immunolabel. In inner medullary CD, principal cells exhibited distinct basolateral KCNQ1 immunolabel, whereas intercalated cells showed diffuse cytoplasmic staining. Thus KCNQ1 protein is widely distributed in mouse distal nephron and CD, with significant axial and cellular heterogeneity in location and intensity. These findings suggest that KCNQ1 has cell-specific roles in renal ion transport and may participate in K(+) secretion and/or absorption along the thick ascending limb, DCT, connecting tubule, and CD.

Tyrosine kinase and phosphatase regulation of slow delayed-rectifier K+ current in guinea-pig ventricular myocytes

The objective of this study was to investigate the involvement of tyrosine phosphorylation in the regulation of the cardiac slowly activating delayed-rectifier K(+) current (I(Ks)) that is important for action potential repolarization. Constitutive I(Ks) recorded from guinea-pig ventricular myocytes was suppressed by broad-spectrum tyrosine kinase (TK) inhibitors tyrphostin A23 (IC(50), 4.1+/-0.6 microm), tyrphostin A25 (IC(50), 12.1+/-2.1 microm) and genistein (IC(50), 64+/-4 microm), but was relatively insensitive to the inactive analogues tyrphostin A1, tyrphostin A63, daidzein and genistin. I(Ks) was unaffected by AG1478 (10 microm), an inhibitor of epidermal growth factor receptor TK, and was strongly suppressed by the Src TK inhibitor PP2 (10 microm) but not by the inactive analogue PP3 (10 microm). The results of experiments with forskolin, H89 and bisindolylmaleimide I indicate that the suppression of I(Ks) by TK inhibitors was not mediated via inhibition of (I(Ks)-stimulatory) protein kinases A and C. To evaluate whether the suppression was related to lowered tyrosine phosphorylation, myocytes were pretreated with TK inhibitors and then exposed to the phosphotyrosyl phosphatase inhibitor orthovanadate (1 mm). Orthovanadate almost completely reversed the suppression of I(Ks) induced by broad-spectrum TK inhibitors at concentrations around their IC(50) values. We conclude that basal I(Ks) is strongly dependent on tyrosine phosphorylation of Ks channel (or channel-regulatory) protein.

The KCNQ1 potassium channel: from gene to physiological function

The voltage-gated KCNQ1 (KvLQT1, Kv7.1) potassium channel plays a crucial role in shaping the cardiac action potential as well as in controlling the water and salt homeostasis in several epithelial tissues. KCNQ1 channels in these tissues are tightly regulated by auxiliary proteins and accessory factors, capable of modulating the properties of the channel complexes. This paper reviews the current knowledge about the KCNQ1 channel with a major focus on interacting proteins and physiological functions.

The corticosteroid hormone induced factor: a new modulator of KCNQ1 channels?

The corticosteroid hormone induced factor (CHIF) is a member of the one-transmembrane segment protein family named FXYD, which also counts phospholemman and the Na,K-pump gamma-subunit. Originally it was suggested that CHIF could induce the expression of the I(Ks) current when expressed in Xenopus laevis oocytes, but recently CHIF has attracted attention as a modulatory subunit of the Na,K-pump. In renal and intestinal epithelia, the expression of CHIF is dramatically up-regulated in response to aldosterone stimulation, and regulation of epithelial ion channels by CHIF is an attractive hypothesis. To study a potential regulatory effect of the CHIF subunit on KCNQ1 channels, co-expression experiments were performed in Xenopus laevis oocytes and mammalian CHO-K1 cells. Electrophysiological characterization was obtained by two-electrode voltage-clamp and patch-clamp, respectively. In both expression systems, we find that CHIF drastically modulates the KCNQ1 current; in the presence of CHIF, the KCNQ1 channels open at all membrane potentials. Thereby, CHIF is the first accessory subunit shown to be capable of modulating both the Na,K-pump and an ion channel. To find a possible physiological function of the constitutively open KCNQ1/CHIF complex, the precise localization of KCNQ1 and CHIF in distal colon and kidney from control and salt-depleted rats was determined by confocal microscopy. However, in these tissues, we did not detect an obvious overlap in expression between KCNQ1 and CHIF. In conclusion, the hormone-regulated subunit CHIF modulates the voltage sensitivity of the KCNQ channels, but so far evidence for an actual co-localization of CHIF and KCNQ1 channels in native tissue is lacking.

Basolateral localization of native ClC-2 chloride channels in absorptive intestinal epithelial cells and basolateral sorting encoded by a CBS-2 domain di-leucine motif

The Cl– channel ClC-2 is expressed in transporting epithelia and has been proposed as an alternative route for Cl– efflux that might compensate for the malfunction of CFTR in cystic fibrosis. There is controversy concerning the cellular and membrane location of ClC-2, particularly in intestinal tissue. The aim of this paper is to resolve this controversy by immunolocalization studies using tissues from ClC-2 knockout animals as control, ascertaining the sorting of ClC-2 in model epithelial cells and exploring the possible molecular signals involved in ClC-2 targeting. ClC-2 was exclusively localized at the basolateral membranes of surface colonic cells or villus duodenal enterocytes. ClC-2 was sorted to the basolateral membranes in MDCK, Caco-2 and LLC-PK1-μ1B, but not in LLC-PK1-μ1A cells. Mutating a di-leucine motif (L812L813) to a di-alanine changed the basolateral targeting of ClC-2 to an apical location. The basolateral membrane localization of ClC-2 in absorptive cells of the duodenum and the colon is compatible with an absorptive function for this Cl– channel. Basolateral targeting information is contained in a di-leucine motif (L812L813) within CBS-2 domain at the C-terminus of ClC-2. It is speculated that ClC-2 also contains an apical sorting signal masked by L812L813. The proposal that CBS domains in ClC channels might behave as regulatory sites sensing intracellular signals opens an opportunity for pharmacological modulation of ClC-2 targeting.

Trafficking of potassium channels

Recent progress in our understanding of the trafficking of potassium channels can be seen in particular when considering the Kv-type channels. To date, we have discovered that folding of the Kv1.3 T1 domain begins in the ribosomal exit tunnel, and that the cell surface expression of Kv4 channels is enhanced by the presence of two recently identified accessory subunits. Current advances are beginning to enable us to understand the Kv supermolecular complex containing these subunits in crystallographic detail. In addition, determinants that govern the dendritic or axonal targeting of Kv channels have also been identified. In terms of the bigger picture, the careful analysis of gene expression patterns in the brain paves the way for studying trafficking in a physiological context. Indeed, neuronal activity has recently been shown to fine-tune the localization of Kv2.1 channels in microdomains of the neuronal plasma membrane.

hKCNE4 inhibits the hKCNQ1 potassium current without affecting the activation kinetics

All five members of the KCNE beta-subunit family are capable of modulating the KCNQ1 potassium current. We have previously published that the murine variant of KCNE4 inhibits the human KCNQ1 current [J. Physiol. 542 (2002) 119]. Recently, this finding has been challenged by Teng et al., stating that the human variant of KCNE4 does not attenuate the KCNQ1 current but does slightly modulate the activation kinetics of the channel after expression in Xenopus laevis oocytes [Biochem. Biophys. Res. Commun. 303 (2003) 808]. In the present study, a detailed investigation on the ability of human and murine KCNE4 to affect either human or murine KCNQ1 currents has been performed. We find that the hKCNE4 subunit drastically inhibits the hKCNQ1 current after expression in X. laevis oocytes. This inhibitory effect is also observed for both hKCNE4 and mKCNE4 when either of these subunits is co-expressed with mKCNQ1. Analyses of the current properties of hKCNQ1 revealed that activation kinetics are independent of the presence of hKCNE4. hKCNE4 has, however, the ability to prevent the inactivation observed for the KCNQ1 current. Based upon previous studies and the present results, it is concluded that both hKCNE4 and mKCNE4 have a drastic inhibitory impact on both hKCNQ1 and mKCNQ1 currents.