MinK-KvLQT1 fusion proteins, evidence for multiple stoichiometries of the assembled IsK channel

The fully activated open state of KCNQ1 controls the cardiac "fight-or-flight" response

The cardiac KCNQ1+KCNE1 (I Ks ) channel regulates heart rhythm in both normal and stress conditions. Under stress, the β-adrenergic stimulation elevates the intracellular cAMP level, leading to KCNQ1 phosphorylation by protein kinase A and increased I Ks , which shortens action potentials to adapt to accelerated heart rate. An impaired response to the β-adrenergic stimulation due to KCNQ1 mutations is associated with the occurrence of a lethal congenital long QT syndrome (type 1, also known as LQT1). However, the underlying mechanism of β-adrenergic stimulation of I Ks remains unclear, impeding the development of new therapeutics. Here we find that the unique properties of KCNQ1 channel gating with two distinct open states are key to this mechanism. KCNQ1's fully activated open (AO) state is more sensitive to cAMP than its' intermediate open (IO) state. By enhancing the AO state occupancy, the small molecules ML277 and C28 are found to effectively enhance the cAMP sensitivity of the KCNQ1 channel, independent of KCNE1 association. This finding of enhancing AO state occupancy leads to a potential novel strategy to rescue the response of I Ks to β-adrenergic stimulation in LQT1 mutants. The success of this approach is demonstrated in cardiac myocytes and also in a high-risk LQT1 mutation. In conclusion the present study not only uncovers the key role of the AO state in I Ks channel phosphorylation, but also provides a new target for anti-arrhythmic strategy.

Significance statement

The increase of I Ks potassium currents with adrenalin stimulation is important for "fight-or-flight" responses. Mutations of the IKs channel reducing adrenalin responses are associated with more lethal form of the type-1 long-QT syndrome (LQT). The alpha subunit of the IKs channel, KCNQ1 opens in two distinct open states, the intermediate-open (IO) and activated-open (AO) states, following a two-step voltage sensing domain (VSD) activation process. We found that the AO state, but not the IO state, is responsible for the adrenalin response. Modulators that specifically enhance the AO state occupancy can enhance adrenalin responses of the WT and LQT-associated mutant channels. These results reveal a mechanism of state dependent modulation of ion channels and provide an anti-arrhythmic strategy.

Mechanistic insights into robust cardiac I Ks potassium channel activation by aromatic polyunsaturated fatty acid analogues

Voltage-gated potassium (K _V ) channels are important regulators of cellular excitability and control action potential repolarization in the heart and brain. K _V channel mutations lead to disordered cellular excitability. Loss-of-function mutations, for example, result in membrane hyperexcitability, a characteristic of epilepsy and cardiac arrhythmias. Interventions intended to restore K _V channel function have strong therapeutic potential in such disorders. Polyunsaturated fatty acids (PUFAs) and PUFA analogues comprise a class of K _V channel activators with potential applications in the treatment of arrhythmogenic disorders such as Long QT Syndrome (LQTS). LQTS is caused by a loss-of-function of the cardiac I _Ks channel - a tetrameric potassium channel complex formed by K _V 7.1 and associated KCNE1 protein subunits. We have discovered a set of aromatic PUFA analogues that produce robust activation of the cardiac I _Ks channel and a unique feature of these PUFA analogues is an aromatic, tyrosine head group. We determine the mechanisms through which tyrosine PUFA analogues exert strong activating effects on the I _Ks channel by generating modified aromatic head groups designed to probe cation-pi interactions, hydrogen bonding, and ionic interactions. We found that tyrosine PUFA analogues do not activate the I _Ks channel through cation-pi interactions, but instead do so through a combination of hydrogen bonding and ionic interactions.

Modulation of the IKS channel by PIP2 requires two binding sites per monomer

The phosphatidyl-inositol-4,5-bisphosphate (PIP₂) lipid has been shown to be crucial for the coupling between the voltage sensor and the pore of the potassium voltage-gated K_V7 channel family, especially the K_V7.1 channel. Expressed in the myocardium membrane, K_V7.1 forms a complex with KCNE1 auxiliary subunits to generate the I_KS current. Here we present molecular models of the transmembrane region of this complex in its three known states, namely the Resting/Closed (RC), the Intermediate/Closed (IC), and the Activated/Open (AO), robustness of which is assessed by agreement with a range of biophysical data. Molecular Dynamics (MD) simulations of these models embedded in a lipid bilayer including phosphatidyl-inositol-4,5-bisphosphate (PIP₂) lipids show that in presence of KCNE1, two PIP₂ lipids are necessary to stabilize each state. The simulations also show that KCNE1 interacts with both PIP₂ binding sites, forming a tourniquet around the pore and preventing its opening. The present investigation provides therefore key molecular elements that govern the role of PIP₂ in KCNE1 modulation of I_KS channels, possibly a common mechanism by which auxiliary KCNE subunits might modulate a variety of other ion channels.

Pharmacological rescue of specific long QT variants of KCNQ1/KCNE1 channels

The congenital Long QT Syndrome (LQTS) is an inherited disorder in which cardiac ventricular repolarization is delayed and predisposes patients to cardiac arrhythmias and sudden cardiac death. LQT1 and LQT5 are LQTS variants caused by mutations in KCNQ1 or KCNE1 genes respectively. KCNQ1 and KCNE1 co-assemble to form critical I_KS potassium channels. Beta-blockers are the standard of care for the treatment of LQT1, however, doing so based on mechanisms other than correcting the loss-of-function of K⁺ channels. ML277 and R-L3 are compounds that enhance I_KS channels and slow channel deactivation in a manner that is dependent on the stoichiometry of KCNE1 subunits in the assembled channels. In this paper, we used expression of I_KS channels in Chinese hamster ovary (CHO) cells and Xenopus oocytes to study the potential of these two drugs (ML277 and R-L3) for the rescue of LQT1 and LQT5 mutant channels. We focused on the LQT1 mutation KCNQ1-S546L, and two LQT5 mutations, KCNE1-L51H and KCNE1-G52R. We found ML277 and R-L3 potentiated homozygote LQTS mutations in the I_KS complexes-KCNE1-G52R and KCNE1-L51H and in heterogeneous I_KS channel complexes which mimic heterogeneous expression of mutations in patients. ML277 and R-L3 increased the mutant I_KS current amplitude and slowed current deactivation, but not in wild type (WT) I_KS. We obtained similar results in the LQT1 mutant (KCNQ1 S546L/KCNE1) with ML277 and R-L3. ML277 and R-L3 had a similar effect on the LQT1 and LQT5 mutants, however, ML277 was more effective than R-L3 in this modulation. Importantly we found that not all LQT5 mutants expressed with KCNQ1 resulted in channels that are potentiated by these drugs as the KCNE1 mutant D76N inhibited drug action when expressed with KCNQ1. Thus, our work shows that by directly studying the treatment of LQT1 and LQT5 mutations with ML277 and R-L3, we will understand the potential utility of these activators as options in specific LQTS therapeutics.

Membrane pools of phosphatidylinositol-4-phosphate regulate KCNQ1/KCNE1 membrane expression

Plasma membrane phosphatidylinositol 4-phosphate (PI4P) is a precursor of PI(4,5)P₂, an important regulator of a large number of ion channels. Although the role of the phospholipid PI(4,5)P₂ in stabilizing ion channel function is well established, little is known about the role of phospholipids in channel membrane localization and specifically the role of PI4P in channel function and localization. The phosphatidylinositol 4-kinases (PI4Ks) synthesize PI4P. Our data show that inhibition of PI4K and prolonged decrease of levels of plasma membrane PI4P lead to a decrease in the KCNQ1/KCNE1 channel membrane localization and function. In addition, we show that mutations linked to Long QT syndrome that affect channel interactions with phospholipids lead to a decrease in membrane expression. We show that expression of a LQT1-associated C-terminal deletion mutant abolishes PI4Kinase-mediated decrease in membrane expression and rescues membrane expression for phospholipid-targeting mutations. Our results indicate a novel role for PI4P on ion channel regulation. Our data suggest that decreased membrane PI4P availability to the channel, either due to inhibition of PI4K or as consequence of mutations, dramatically inhibits KCNQ1/KCNE1 channel membrane localization and current. Our results may have implications to regulation of other PI4P binding channels.

Compound Heterozygous KCNQ1 Mutations Causing Recessive Romano-Ward Syndrome: Functional Characterization by Mutant Co-expression

Next Generation Sequencing has identified many KCNQ1 genetic variants associated with type 1 long QT or Romano-Ward syndrome, most frequently inherited in an autosomal dominant fashion, although recessive forms have been reported. Particularly in the case of missense variants, functional studies of mutants are of aid to establish variant pathogenicity and to understand the mechanistic basis of disease. Two compound heterozygous KCNQ1 mutations (p.A300T and p.P535T) were previously found in a child who suffered sudden death. To provide further insight into the clinical significance and basis for pathogenicity of these variants, different combinations of wildtype, A300T and P535T alleles were co-expressed with the accessory β-subunit minK in HEK293 cells, to analyze colocalization with the plasma membrane and some biophysical phenotypes of homo and heterotetrameric channels using the patch-clamp technique. A300T homotetrameric channels showed left-shifted activation V_1/2 as previously observed in Xenopus oocytes, decreased maximum conductance density, slow rise-time_300ms, and a characteristic use-dependent response. A300T slow rise-time_300ms and use-dependent response behaved as dominant biophysical traits for all allele combinations. The P535T variant significantly decreased maximum conductance density and Kv7.1-minK-plasma membrane colocalization. P535T/A300T heterotetrameric channels showed decreased colocalization with plasma membrane, slow rise-time_300ms and the A300T characteristic use-dependent response. While A300T left shifted activation voltage dependence behaved as a recessive trait when co-expressed with WT alleles, it was dominant when co-expressed with P535T alleles. Conclusions: The combination of P535T/A300T channel biophysical properties is compatible with recessive Romano Ward syndrome. Further analysis of other biophysical traits may identify other mechanisms involved in the pathophysiology of this disease.

The auxiliary subunit KCNE1 regulates KCNQ1 channel response to sustained calcium-dependent PKC activation

The slow cardiac delayed rectifier current (IKs) is formed by KCNQ1 and KCNE1 subunits and is one of the major repolarizing currents in the heart. Decrease of IKs currents either due to inherited mutations or pathological remodeling is associated with increased risk for cardiac arrhythmias and sudden death. Ca2+-dependent PKC isoforms (cPKC) are chronically activated in heart disease and diabetes. Recently, we found that sustained stimulation of the calcium-dependent PKCβII isoform leads to decrease in KCNQ1 subunit membrane localization and KCNQ1/KCNE1 channel activity, although the role of KCNE1 in this regulation was not explored. Here, we show that the auxiliary KCNE1 subunit expression is necessary for channel internalization. A mutation in a KCNE1 phosphorylation site (KCNE1(S102A)) abolished channel internalization in both heterologous expression systems and cardiomyocytes. Altogether, our results suggest that KCNE1(S102) phosphorylation by PKCβII leads to KCNQ1/KCNE1 channel internalization in response to sustained PKC stimulus, while leaving KCNQ1 homomeric channels in the membrane. This preferential internalization is expected to have strong impact on cardiac repolarization. Our results suggest that KCNE1(S102) is an important anti-arrhythmic drug target to prevent IKs pathological remodeling leading to cardiac arrhythmias.

Hormonal Signaling Actions on Kv7.1 (KCNQ1) Channels

Kv7 channels (Kv7.1-7.5) are voltage-gated K⁺ channels that can be modulated by five β-subunits (KCNE1-5). Kv7.1-KCNE1 channels produce the slow-delayed rectifying K⁺ current, I_Ks, which is important during the repolarization phase of the cardiac action potential. Kv7.2-7.5 are predominantly neuronally expressed and constitute the muscarinic M-current and control the resting membrane potential in neurons. Kv7.1 produces drastically different currents as a result of modulation by KCNE subunits. This flexibility allows the Kv7.1 channel to have many roles depending on location and assembly partners. The pharmacological sensitivity of Kv7.1 channels differs from that of Kv7.2-7.5 and is largely dependent upon the number of β-subunits present in the channel complex. As a result, the development of pharmaceuticals targeting Kv7.1 is problematic. This review discusses the roles and the mechanisms by which different signaling pathways affect Kv7.1 and KCNE channels and could potentially provide different ways of targeting the channel.

Gating and Regulation of KCNQ1 and KCNQ1 + KCNE1 Channel Complexes

The IKs channel complex is formed by the co-assembly of Kv7.1 (KCNQ1), a voltage-gated potassium channel, with its β-subunit, KCNE1 and the association of numerous accessory regulatory molecules such as PIP2, calmodulin, and yotiao. As a result, the IKs potassium current shows kinetic and regulatory flexibility, which not only allows IKs to fulfill physiological roles as disparate as cardiac repolarization and the maintenance of endolymph K⁺ homeostasis, but also to cause significant disease when it malfunctions. Here, we review new areas of understanding in the assembly, kinetics of activation and inactivation, voltage-sensor pore coupling, unitary events and regulation of this important ion channel complex, all of which have been given further impetus by the recent solution of cryo-EM structural representations of KCNQ1 alone and KCNQ1+KCNE3. Recently, the stoichiometric ratio of KCNE1 to KCNQ1 subunits has been confirmed to be variable up to a ratio of 4:4, rather than fixed at 2:4, and we will review the results and new methodologies that support this conclusion. Significant advances have been made in understanding differences between KCNQ1 and IKs gating using voltage clamp fluorimetry and mutational analysis to illuminate voltage sensor activation and inactivation, and the relationship between voltage sensor translation and pore domain opening. We now understand that the KCNQ1 pore can open with different permeabilities and conductance when the voltage sensor is in partially or fully activated positions, and the ability to make robust single channel recordings from IKs channels has also revealed the complicated pore subconductance architecture during these opening steps, during inactivation, and regulation by 1−4 associated KCNE1 subunits. Experiments placing mutations into individual voltage sensors to drastically change voltage dependence or prevent their movement altogether have demonstrated that the activation of KCNQ1 alone and IKs can best be explained using allosteric models of channel gating. Finally, we discuss how the intrinsic gating properties of KCNQ1 and IKs are highly modulated through the impact of intracellular signaling molecules and co-factors such as PIP2, protein kinase A, calmodulin and ATP, all of which modulate IKs current kinetics and contribute to diverse IKs channel complex function.

Functional Consequences of the Variable Stoichiometry of the Kv1.3-KCNE4 Complex

The voltage-gated potassium channel Kv1.3 plays a crucial role during the immune response. The channel forms oligomeric complexes by associating with several modulatory subunits. KCNE4, one of the five members of the KCNE family, binds to Kv1.3, altering channel activity and membrane expression. The association of KCNEs with Kv channels is the subject of numerous studies, and the stoichiometry of such associations has led to an ongoing debate. The number of KCNE4 subunits that can interact and modulate Kv1.3 is unknown. KCNE4 transfers important elements to the Kv1.3 channelosome that negatively regulate channel function, thereby fine-tuning leukocyte physiology. The aim of this study was to determine the stoichiometry of the functional Kv1.3-KCNE4 complex. We demonstrate that as many as four KCNE4 subunits can bind to the same Kv1.3 channel, indicating a variable Kv1.3-KCNE4 stoichiometry. While increasing the number of KCNE4 subunits steadily slowed the activation of the channel and decreased the abundance of Kv1.3 at the cell surface, the presence of a single KCNE4 peptide was sufficient for the cooperative enhancement of the inactivating function of the channel. This variable architecture, which depends on KCNE4 availability, differentially affects Kv1.3 function. Therefore, our data indicate that the physiological remodeling of KCNE4 triggers functional consequences for Kv1.3, thus affecting cell physiology.

Gating modulation of the KCNQ1 channel by KCNE proteins studied by voltage-clamp fluorometry

The KCNQ1 channel is a voltage-dependent potassium channel and is ubiquitously expressed throughout the human body including the heart, lung, kidney, pancreas, intestine and inner ear. Gating properties of the KCNQ1 channel are modulated by KCNE auxiliary subunits. For example, the KCNQ1-KCNE1 channel produces a slowly-activating potassium current, while KCNE3 makes KCNQ1 a voltage-independent, constitutively open channel. Thus, physiological functions of KCNQ1 channels are greatly dependent on the type of KCNE protein that is co-expressed in that organ. It has long been debated how the similar single transmembrane KCNE proteins produce quite different gating behaviors. Recent applications of voltage-clamp fluorometry (VCF) for the KCNQ1 channel have shed light on this question. The VCF is a quite sensitive method to detect structural changes of membrane proteins and is especially suitable for tracking the voltage sensor domains of voltage-gated ion channels. In this short review, I will introduce how the VCF technique can be applied to detect structural changes and what have been revealed by the recent VCF applications to the gating modulation of KCNQ1 channels by KCNE proteins.

I Ks ion-channel pore conductance can result from individual voltage sensor movements

The I _Ks current has an established role in cardiac action potential repolarization, and provides a repolarization reserve at times of stress. The underlying channels are formed from tetramers of KCNQ1 along with one to four KCNE1 accessory subunits, but how these components together gate the I _Ks complex to open the pore is controversial. Currently, either a concerted movement involving all four subunits of the tetramer or allosteric regulation of open probability through voltage-dependent subunit activation is thought to precede opening. Here, by using the E160R mutation in KCNQ1 or the F57W mutation in KCNE1 to prevent or impede, respectively, voltage sensors from moving into activated conformations, we demonstrate that a concerted transition of all four subunits after voltage sensor activation is not required for the opening of I _Ks channels. Tracking voltage sensor movement, via [2-(trimethylammonium)ethyl]methanethiosulfonate bromide (MTSET) modification and fluorescence recordings, shows that E160R-containing voltage sensors do not translocate upon depolarization. E160R, when expressed in all four KCNQ1 subunits, is nonconducting, but if one, two, or three voltage sensors contain the E160R mutation, whole-cell and single-channel currents are still observed in both the presence and absence of KCNE1, and average conductance is reduced proportional to the number of E160R voltage sensors. The data suggest that KCNQ1 + KCNE1 channels gate like KCNQ1 alone. A model of independent voltage sensors directly coupled to open states can simulate experimental changes in I _Ks current kinetics, including the nonlinear depolarization of the conductance-voltage (G-V) relationship, and tail current acceleration as the number of nonactivatable E160R subunits is increased.

The IKs Channel Response to cAMP Is Modulated by the KCNE1:KCNQ1 Stoichiometry

The delayed potassium rectifier current, I_Ks, is assembled from tetramers of KCNQ1 and varying numbers of KCNE1 accessory subunits in addition to calmodulin. This channel complex is important in the response of the cardiac action potential to sympathetic stimulation, during which I_Ks is enhanced. This is likely due to channels opening more quickly, more often, and to greater sublevel amplitudes during adrenergic stimulation. KCNQ1 alone is unresponsive to cyclic adenosine monophosphate (cAMP), and thus KCNE1 is required for a functional effect of protein kinase A phosphorylation. Here, we investigate the effect that KCNE1 has on the response to 8-4-chlorophenylthio (CPT)-cAMP, a membrane-permeable cAMP analog, by varying the number of KCNE1 subunits present using fusion constructs of I_Ks with either one (EQQQQ) or two (EQQ) KCNE1 subunits in the channel complex with KCNQ1. These experiments use both whole-cell and single-channel recording techniques. EQQ (2:4, E1:Q1) shows a significant shift in V_1/2 of activation from 10.4 mV ± 2.2 in control to -2.7 mV ± 1.2 (p-value: 0.0024). EQQQQ (1:4, E1:Q1) shows a smaller change in response to 8-CPT-cAMP, 6.3 mV ± 2.3 to -3.2 mV ± 3.0 (p-value: 0.0435). As the number of KCNE1 subunits is reduced, the shift in the V_1/2 of activation becomes smaller. At the single-channel level, a similar graded change in subconductance occupancy and channel activity is seen in response to 8-CPT-cAMP: the less E1, the smaller the response. However, both constructs show a significant reduction of a similar magnitude in the first latency to opening (EQQ control: 0.90 s ± 0.07 to 0.71 s ± 0.06, p-value: 0.0032 and EQQQQ control: 0.94 s ± 0.09 to 0.56 s ± 0.07, p-value < 0.0001). This suggests that there are both E1-dependent and E1-independent effects of 8-CPT-cAMP on the channel.

Transcripts of Kv7.1 and MinK channels and slow delayed rectifier K+ current (IKs) are expressed in zebrafish (Danio rerio) heart

Zebrafish are increasingly used as a model for human cardiac electrophysiology, arrhythmias, and drug screening. However, K⁺ ion channels of the zebrafish heart, which determine the rate of repolarization and duration of cardiac action potential (AP) are still incompletely known and characterized. Here, we provide the first evidence for the presence of the slow component of the delayed rectifier K⁺channels in the zebrafish heart and characterize electrophysiological properties of the slow component of the delayed rectifier K⁺current, I_Ks. Zebrafish atrium and ventricle showed strong transcript expression of the kcnq1 gene, which encodes the Kv7.1 α-subunit of the slow delayed rectifier K⁺ channel. In contrast, the kcne1 gene, encoding the MinK β-subunit of the delayed rectifier, was expressed at 21 and 17 times lower level in ventricle and atrium, respectively, in comparison to the kcnq1. I_Ks was observed in 62% of ventricular myocytes with mean (± SEM) density of 1.23 ± 0.37 pA/pF at + 30 mV. Activation rate of I_Ks was 38% faster (τ₅₀ = 1248 ± 215 ms) than kcnq1:kcne1 channels (1725 ± 792 ms) expressed in 3:1 ratio in Chinese hamster ovary cells. Microelectrode experiments demonstrated the functional relevance of I_Ks in the zebrafish heart, since 100 μM chromanol 293B produced a significant prolongation of AP in zebrafish ventricle. We conclude that AP repolarization in zebrafish ventricle is contributed by I_Ks, which is mainly generated by homotetrameric Kv7.1 channels not coupled to MinK ancillary β-subunits. This is a clear difference to the human heart, where MinK is an essential component of the slow delayed rectifier K⁺channel.

Phosphatidylinositol-4,5-bisphosphate is required for KCNQ1/KCNE1 channel function but not anterograde trafficking

The slow delayed-rectifier potassium current (I_Ks) is crucial for human cardiac action potential repolarization. The formation of I_Ks requires co-assembly of the KCNQ1 α-subunit and KCNE1 β-subunit, and mutations in either of these subunits can lead to hereditary long QT syndrome types 1 and 5, respectively. It is widely recognised that the KCNQ1/KCNE1 (Q1/E1) channel requires phosphatidylinositol-4,5-bisphosphate (PIP₂) binding for function. We previously identified a cluster of basic residues in the proximal C-terminus of KCNQ1 that form a PIP₂/phosphoinositide binding site. Upon charge neutralisation of these residues we found that the channel became more retained in the endoplasmic reticulum, which raised the possibility that channel–phosphoinositide interactions could play a role in channel trafficking. To explore this further we used a chemically induced dimerization (CID) system to selectively deplete PIP₂ and/or phosphatidylinositol-4-phosphate (PI(4)P) at the plasma membrane (PM) or Golgi, and we subsequently monitored the effects on both channel trafficking and function. The depletion of PIP₂ and/or PI(4)P at either the PM or Golgi did not alter channel cell-surface expression levels. However, channel function was extremely sensitive to the depletion of PIP₂ at the PM, which is in contrast to the response of other cardiac potassium channels tested (Kir2.1 and Kv11.1). Surprisingly, when using the CID system I_Ks was dramatically reduced even before dimerization was induced, highlighting limitations regarding the utility of this system when studying processes highly sensitive to PIP₂ depletion. In conclusion, we identify that the Q1/E1 channel does not require PIP₂ or PI(4)P for anterograde trafficking, but is heavily reliant on PIP₂ for channel function once at the PM.

Chansporter complexes in cell signaling

Ion channels facilitate diffusion of ions across cell membranes for such diverse purposes as neuronal signaling, muscular contraction, and fluid homeostasis. Solute transporters often utilize ionic gradients to move aqueous solutes up their concentration gradient, also fulfilling a wide variety of tasks. Recently, an increasing number of ion channel-transporter ('chansporter') complexes have been discovered. Chansporter complex formation may overcome what could otherwise be considerable spatial barriers to rapid signal integration and feedback between channels and transporters, the ions and other substrates they transport, and environmental factors to which they must respond. Here, current knowledge in this field is summarized, covering both heterologous expression structure/function findings and potential mechanisms by which chansporter complexes fulfill contrasting roles in cell signaling in vivo.

Gating mechanisms underlying deactivation slowing by two KCNQ1 atrial fibrillation mutations

KCNQ1 is a voltage-gated potassium channel that is modulated by the beta-subunit KCNE1 to generate I_Ks, the slow delayed rectifier current, which plays a critical role in repolarizing the cardiac action potential. Two KCNQ1 gain-of-function mutations that cause a genetic form of atrial fibrillation, S140G and V141M, drastically slow I_Ks deactivation. However, the underlying gating alterations remain unknown. Voltage clamp fluorometry (VCF) allows simultaneous measurement of voltage sensor movement and current through the channel pore. Here, we use VCF and kinetic modeling to determine the effects of mutations on channel voltage-dependent gating. We show that in the absence of KCNE1, S140G, but not V141M, directly slows voltage sensor movement, which indirectly slows current deactivation. In the presence of KCNE1, both S140G and V141M slow pore closing and alter voltage sensor-pore coupling, thereby slowing current deactivation. Our results suggest that KCNE1 can mediate changes in pore movement and voltage sensor-pore coupling to slow I_Ks deactivation and provide a key step toward developing mechanism-based therapies.

Ubiquitylation-dependent oligomerization regulates activity of Nedd4 ligases

Ubiquitylation controls protein function and degradation. Therefore, ubiquitin ligases need to be tightly controlled. We discovered an evolutionarily conserved allosteric restraint mechanism for Nedd4 ligases and demonstrated its function with diverse substrates: the yeast soluble proteins Rpn10 and Rvs167, and the human receptor tyrosine kinase FGFR1 and cardiac I_KS potassium channel. We found that a potential trimerization interface is structurally blocked by the HECT domain α1-helix, which further undergoes ubiquitylation on a conserved lysine residue. Genetic, bioinformatics, biochemical and biophysical data show that attraction between this α1-conjugated ubiquitin and the HECT ubiquitin-binding patch pulls the α1-helix out of the interface, thereby promoting trimerization. Strikingly, trimerization renders the ligase inactive. Arginine substitution of the ubiquitylated lysine impairs this inactivation mechanism and results in unrestrained FGFR1 ubiquitylation in cells. Similarly, electrophysiological data and TIRF microscopy show that NEDD4 unrestrained mutant constitutively downregulates the I_KS channel, thus confirming the functional importance of E3-ligase autoinhibition.

Molecular Pathophysiology of Congenital Long QT Syndrome

Ion channels represent the molecular entities that give rise to the cardiac action potential, the fundamental cellular electrical event in the heart. The concerted function of these channels leads to normal cyclical excitation and resultant contraction of cardiac muscle. Research into cardiac ion channel regulation and mutations that underlie disease pathogenesis has greatly enhanced our knowledge of the causes and clinical management of cardiac arrhythmia. Here we review the molecular determinants, pathogenesis, and pharmacology of congenital Long QT Syndrome. We examine mechanisms of dysfunction associated with three critical cardiac currents that comprise the majority of congenital Long QT Syndrome cases: 1) IKs, the slow delayed rectifier current; 2) IKr, the rapid delayed rectifier current; and 3) INa, the voltage-dependent sodium current. Less common subtypes of congenital Long QT Syndrome affect other cardiac ionic currents that contribute to the dynamic nature of cardiac electrophysiology. Through the study of mutations that cause congenital Long QT Syndrome, the scientific community has advanced understanding of ion channel structure-function relationships, physiology, and pharmacological response to clinically employed and experimental pharmacological agents. Our understanding of congenital Long QT Syndrome continues to evolve rapidly and with great benefits: genotype-driven clinical management of the disease has improved patient care as precision medicine becomes even more a reality.

PIP Water Transport and Its pH Dependence Are Regulated by Tetramer Stoichiometry

Many plasma membrane channels form oligomeric assemblies, and heterooligomerization has been described as a distinctive feature of some protein families. In the particular case of plant plasma membrane aquaporins (PIPs), PIP1 and PIP2 monomers interact to form heterotetramers. However, the biological properties of the different heterotetrameric configurations formed by PIP1 and PIP2 subunits have not been addressed yet. Upon coexpression of tandem PIP2-PIP1 dimers in Xenopus oocytes, we can address, for the first time to our knowledge, the functional properties of single heterotetrameric species having 2:2 stoichiometry. We have also coexpressed PIP2-PIP1 dimers with PIP1 and PIP2 monomers to experimentally investigate the localization and biological activity of each tetrameric assembly. Our results show that PIP2-PIP1 heterotetramers can assemble with 3:1, 1:3, or 2:2 stoichiometry, depending on PIP1 and PIP2 relative expression in the cell. All PIP2-PIP1 heterotetrameric species localize at the plasma membrane and present the same water transport capacity. Furthermore, the contribution of any heterotetrameric assembly to the total water transport through the plasma membrane doubles the contribution of PIP2 homotetramers. Our results also indicate that plasma membrane water transport can be modulated by the coexistence of different tetrameric species and by intracellular pH. Moreover, all the tetrameric species present similar cooperativity behavior for proton sensing. These findings throw light on the functional properties of PIP tetramers, showing that they have flexible stoichiometry dependent on the quantity of PIP1 and PIP2 molecules available. This represents, to our knowledge, a novel regulatory mechanism to adjust water transport across the plasma membrane.

KCNE4 and KCNE5: K(+) channel regulation and cardiac arrhythmogenesis

KCNE proteins are single transmembrane-segment voltage-gated potassium (Kv) channel ancillary subunits that exhibit a diverse range of physiological functions. Human KCNE gene mutations are associated with various pathophysiological states, most notably cardiac arrhythmias. Of the five isoforms in the human KCNE gene family, KCNE4 and the X-linked KCNE5 are, to date, the least-studied. Recently, however, interest in these neglected genes has been stoked by their putative association with debilitating or lethal cardiac arrhythmias. The sometimes-overlapping functional effects of KCNE4 and KCNE5 vary depending on both their Kv α subunit partner and on other ancillary subunits within the channel complex, but mostly fall into two contrasting categories - either inhibition, or fine-tuning of gating kinetics. This review covers current knowledge regarding the molecular mechanisms of KCNE4 and KCNE5 function, human disease associations, and findings from very recent studies of cardiovascular pathophysiology in Kcne4(-/-) mice.

Unnatural amino acid photo-crosslinking of the IKs channel complex demonstrates a KCNE1:KCNQ1 stoichiometry of up to 4:4

Cardiac repolarization is determined in part by the slow delayed rectifier current (I_Ks), through the tetrameric voltage-gated ion channel, KCNQ1, and its β-subunit, KCNE1. The stoichiometry between α and β-subunits has been controversial with studies reporting either a strict 2 KCNE1:4 KCNQ1 or a variable ratio up to 4:4. We used I_Ks fusion proteins linking KCNE1 to one (EQ), two (EQQ) or four (EQQQQ) KCNQ1 subunits, to reproduce compulsory 4:4, 2:4 or 1:4 stoichiometries. Whole cell and single-channel recordings showed EQQ and EQQQQ to have increasingly hyperpolarized activation, reduced conductance, and shorter first latency of opening compared to EQ - all abolished by the addition of KCNE1. As well, using a UV-crosslinking unnatural amino acid in KCNE1, we found EQQQQ and EQQ crosslinking rates to be progressively slowed compared to KCNQ1, which demonstrates that no intrinsic mechanism limits the association of up to four β-subunits within the I_Ks complex.

DOI:http://dx.doi.org/10.7554/eLife.11815.001

The membrane that surrounds heart muscle cells contains specialized channels that can open and close to control the movements of charged ions into and out of the cell. This ion flow generates the electrical signals that stimulate the heart muscle to contract for each heart beat.

Different ion channels influence different steps in the initiation and termination of each electrical signal. For example, the I_Ks ion channel complex helps to return the cell to a resting state so the heart muscle can relax. This allows chambers of the heart to fill with blood before the next beat pumps blood throughout the body. Mutations that affect I_Ks cause serious heart conditions that affect heart rhythm, such as Long QT Syndrome.

The I_Ks complex consists of channels that are each made of four copies of a protein called KCNQ1, through which potassium ions exit the cell. This channel opens in response to changes in the voltage across the cell membrane (known as the “membrane potential”). A small protein subunit called KCNE1 also makes up part of the complex, but it was not clear how many KCNE1 molecules combine with KCNQ1 to form a working channel complex. Several previous studies have reported two different results: that the KCNQ1 channel complex only exists with two KCNE1 molecules, or that the association is flexible, allowing the complex to contain up to four KCNE1 subunits.

Murray et al. have now constructed I_Ks fusion channels out of different numbers of KCNQ1 and KCNE1 molecules to investigate how different KCNQ1:KCNE1 ratios affect how the channel works. Measuring the responses of these modified channels in mammalian cells revealed that channels with four KCNE1 subunits conducted ions better than channels with one or two KCNE1s. The channels containing fewer KCNE1s also opened at lower membrane potentials and after a shorter delay following a change in the membrane potential. Further experiments also supported the theory that up to four independent KCNE1 subunits may be easily added to the I_Ks ion channel complex.

Murray et al. suggest that by being able to form channel complexes containing different numbers of KCNE1 subunits, cells can more flexibly control the rate at which ions flow out of the heart cells to tune the electrical signals that trigger each heart beat. The next challenges will be to determine the composition of the I_Ks channel complex in adult heart cells and to investigate how the complex might change with disease.

DOI:http://dx.doi.org/10.7554/eLife.11815.002

KCNE1 and KCNE3: The yin and yang of voltage-gated K(+) channel regulation

The human KCNE gene family comprises five genes encoding single transmembrane-spanning ion channel regulatory subunits. The primary function of KCNE subunits appears to be regulation of voltage-gated potassium (Kv) channels, and the best-understood KCNE complexes are with the KCNQ1 Kv α subunit. Here, we review the often opposite effects of KCNE1 and KCNE3 on Kv channel biology, with an emphasis on regulation of KCNQ1. Slow-activating IKs channel complexes formed by KCNQ1 and KCNE1 are essential for human ventricular myocyte repolarization, while constitutively active KCNQ1-KCNE3 channels are important in the intestine. Inherited sequence variants in human KCNE1 and KCNE3 cause cardiac arrhythmias but by different mechanisms, and each is important for hearing in unique ways. Because of their contrasting effects on KCNQ1 function, KCNE1 and KCNE3 have proved invaluable tools in the mechanistic understanding of how channel gating can be manipulated, and each may also provide a window into novel insights and new therapeutic opportunities in K(+) channel pharmacology. Finally, findings from studies of Kcne1(-/-) and Kcne3(-/-) mouse lines serve to illustrate the complexity of KCNE biology and KCNE-linked disease states.

Enhanced effects of isoflurane on the long QT syndrome 1-associated A341V mutant

BACKGROUND

The impact of volatile anesthetics on patients with inherited long QT syndrome (LQTS) is not well understood. This is further complicated by the different genotypes underlying LQTS. No studies have reported on the direct effects of volatile anesthetics on specific LQTS-associated mutations. We investigated the effects of isoflurane on a common LQTS type 1 mutation, A341V, with an unusually severe phenotype.

METHODS

Whole cell potassium currents (IKs) were recorded from HEK293 and HL-1 cells transiently expressing/coexpressing wild-type KCNQ1 (α-subunit), mutant KCNQ1, wild-type KCNE1 (β-subunit), and fusion KCNQ1 + KCNE1. Current was monitored in the absence and presence of clinically relevant concentration of isoflurane (0.54 ± 0.05 mM, 1.14 vol %). Computer simulations determined the resulting impact on the cardiac action potential.

RESULTS

Isoflurane had significantly greater inhibitory effect on A341V + KCNE1 (62.2 ± 3.4%, n = 8) than on wild-type KCNQ1 + KCNE1 (40.7 ± 4.5%; n = 9) in transfected HEK293 cells. Under heterozygous conditions, isoflurane inhibited A341V + KCNQ1 + KCNE1 by 65.2 ± 3.0% (n = 13) and wild-type KCNQ1 + KCNE1 (2:1 ratio) by 32.0 ± 4.5% (n = 11). A341V exerted a dominant negative effect on IKs. Similar differential effects of isoflurane were also observed in experiments using the cardiac HL-1 cells. Mutations of the neighboring F340 residue significantly attenuated the effects of isoflurane, and fusion proteins revealed the modulatory effect of KCNE1. Action potential simulations revealed a stimulation frequency-dependent effect of A341V.

CONCLUSIONS

The LQTS-associated A341V mutation rendered the IKs channel more sensitive to the inhibitory effects of isoflurane compared to wild-type IKs in transfected cell lines; F340 is a key residue for anesthetic action.

The KCNQ1 channel - remarkable flexibility in gating allows for functional versatility

The KCNQ1 channel (also called Kv7.1 or KvLQT1) belongs to the superfamily of voltage-gated K(+) (Kv) channels. KCNQ1 shares several general features with other Kv channels but also displays a fascinating flexibility in terms of the mechanism of channel gating, which allows KCNQ1 to play different physiological roles in different tissues. This flexibility allows KCNQ1 channels to function as voltage-independent channels in epithelial tissues, whereas KCNQ1 function as voltage-activated channels with very slow kinetics in cardiac tissues. This flexibility is in part provided by the association of KCNQ1 with different accessory KCNE β-subunits and different modulators, but also seems like an integral part of KCNQ1 itself. The aim of this review is to describe the main mechanisms underlying KCNQ1 flexibility.

KCNQ1 channel modulation by KCNE proteins via the voltage-sensing domain

The gating of the KCNQ1 potassium channel is drastically regulated by auxiliary subunit KCNE proteins. KCNE1, for example, slows the activation kinetics of KCNQ1 by two orders of magnitude. Like other voltage-gated ion channels, the opening of KCNQ1 is regulated by the voltage-sensing domain (VSD; S1-S4 segments). Although it has been known that KCNE proteins interact with KCNQ1 via the pore domain, some recent reports suggest that the VSD movement may be altered by KCNE. The altered VSD movement of KCNQ1 by KCNE proteins has been examined by site-directed mutagenesis, the scanning cysteine accessibility method (SCAM), voltage clamp fluorometry (VCF) and gating charge measurements. These accumulated data support the idea that KCNE proteins interact with the VSDs of KCNQ1 and modulate the gating of the KCNQ1 channel. In this review, we will summarize recent findings and current views of the KCNQ1 modulation by KCNE via the VSD. In this context, we discuss our recent findings that KCNE1 may alter physical interactions between the S4 segment (VSD) and the S5 segment (pore domain) of KCNQ1. Based on these findings from ourselves and others, we propose a hypothetical mechanism for how KCNE1 binding alters the VSD movement and the gating of the channel.

Domain-domain interactions determine the gating, permeation, pharmacology, and subunit modulation of the IKs ion channel

Voltage-gated ion channels generate electrical currents that control muscle contraction, encode neuronal information, and trigger hormonal release. Tissue-specific expression of accessory (β) subunits causes these channels to generate currents with distinct properties. In the heart, KCNQ1 voltage-gated potassium channels coassemble with KCNE1 β-subunits to generate the I_Ks current (Barhanin et al., 1996; Sanguinetti et al., 1996), an important current for maintenance of stable heart rhythms. KCNE1 significantly modulates the gating, permeation, and pharmacology of KCNQ1 (Wrobel et al., 2012; Sun et al., 2012; Abbott, 2014). These changes are essential for the physiological role of I_Ks (Silva and Rudy, 2005); however, after 18 years of study, no coherent mechanism explaining how KCNE1 affects KCNQ1 has emerged. Here we provide evidence of such a mechanism, whereby, KCNE1 alters the state-dependent interactions that functionally couple the voltage-sensing domains (VSDs) to the pore.

DOI:http://dx.doi.org/10.7554/eLife.03606.001

Cells are surrounded by a membrane that prevents charged molecules from flowing directly into or out of the cell. Instead ions move through channel proteins within the cell membrane. Most ion channel proteins are selective and only allow one or a few types of ion to cross. Ion channels can also be ‘gated’, and have a central pore that can open or close to allow or stop the flow of selected ions. This gating can be affected by the channel sensing changes in conditions, such as changes in the voltage across the cell membrane.

Research conducted more than half a century ago—before the discovery of channel proteins—led to a mathematical model of the flow of potassium ions across a membrane in response to changes in voltage. This model made a number of assumptions, many of which are still widely accepted. However, Zaydman et al. have now called into question some of the assumptions of this model.

Based on the original model, it has been long assumed that the voltage-sensing domains that open or close the central pore in response to changes in voltage must be fully activated to allow the channel to open. It had also been assumed that the voltage-sensing domains do not affect the flow of ions once the channel is open. Zaydman et al. have now shown that these assumptions are not valid for a specific voltage-gated potassium channel called KCNQ1. Instead, this ion channel opens when its voltage-sensing domains are either partially or fully activated. Zaydman found that the intermediate-open and activated-open states had different preferences for passing various types of ion; therefore, the gating of the channel and the flow of ions through the open channel are both dependent on the state of the voltage-sensing domains. This is in direct contrast to what had previously been assumed.

The original model cannot reproduce the gating of KCNQ1, nor can any other established model. Therefore, Zaydman et al. devised a new model to understand how the interactions between different states of the voltage-sensing domains and the pore lead to gating. Zaydman et al. then used their model to address how another protein called KCNE1 is able to alter properties of the KCNQ1 channel.

KCNE1 is a protein that is expressed in the heart muscle cell and mutations affecting KCNQ1 or KCNE1 have been associated with potentially fatal heart conditions. Based on the assumptions of the original model, it had been difficult to understand how KCNE1 was able to affect different properties of the KCNQ1 channel. Thus, for nearly 20 years it has been debated whether KCNE1 primarily affects the activation of the voltage-sensing domains or the opening of the pore. Zaydman et al. found instead that KCNE1 alters the interactions between the voltage-sensing domains and the pore, which prevented the intermediate-open state and modified the properties of the activated-open state. This mechanism provides one of the most complete explanations for the action of the KCNE1 protein.

DOI:http://dx.doi.org/10.7554/eLife.03606.002

Steric hindrance between S4 and S5 of the KCNQ1/KCNE1 channel hampers pore opening

In voltage-gated K(+) channels, membrane depolarization induces an upward movement of the voltage-sensing domains (VSD) that triggers pore opening. KCNQ1 is a voltage-gated K(+) channel and its gating behaviour is substantially modulated by auxiliary subunit KCNE proteins. KCNE1, for example, markedly shifts the voltage dependence of KCNQ1 towards the positive direction and slows down the activation kinetics. Here we identify two phenylalanine residues on KCNQ1, Phe232 on S4 (VSD) and Phe279 on S5 (pore domain) to be responsible for the gating modulation by KCNE1. Phe232 collides with Phe279 during the course of the VSD movement and hinders KCNQ1 channel from opening in the presence of KCNE1. This steric hindrance caused by the bulky amino-acid residues destabilizes the open state and thus shifts the voltage dependence of KCNQ1/KCNE1 channel.

Individual IKs channels at the surface of mammalian cells contain two KCNE1 accessory subunits

KCNE1 (E1) β-subunits assemble with KCNQ1 (Q1) voltage-gated K(+) channel α-subunits to form IKslow (IKs) channels in the heart and ear. The number of E1 subunits in IKs channels has been an issue of ongoing debate. Here, we use single-molecule spectroscopy to demonstrate that surface IKs channels with human subunits contain two E1 and four Q1 subunits. This stoichiometry does not vary. Thus, IKs channels in cells with elevated levels of E1 carry no more than two E1 subunits. Cells with low levels of E1 produce IKs channels with two E1 subunits and Q1 channels with no E1 subunits--channels with one E1 do not appear to form or are restricted from surface expression. The plethora of models of cardiac function, transgenic animals, and drug screens based on variable E1 stoichiometry do not reflect physiology.

A long QT mutation substitutes cholesterol for phosphatidylinositol-4,5-bisphosphate in KCNQ1 channel regulation

Introduction

Phosphatidylinositol-4,5-bisphosphate (PIP₂) is a cofactor necessary for the activity of KCNQ1 channels. Some Long QT mutations of KCNQ1, including R243H, R539W and R555C have been shown to decrease KCNQ1 interaction with PIP₂. A previous study suggested that R539W is paradoxically less sensitive to intracellular magnesium inhibition than the WT channel, despite a decreased interaction with PIP₂. In the present study, we confirm this peculiar behavior of R539W and suggest a molecular mechanism underlying it.

Methods and Results

COS-7 cells were transfected with WT or mutated KCNE1-KCNQ1 channel, and patch-clamp recordings were performed in giant-patch, permeabilized-patch or ruptured-patch configuration. Similar to other channels with a decreased PIP₂ affinity, we observed that the R243H and R555C mutations lead to an accelerated current rundown when membrane PIP₂ levels are decreasing. As opposed to R243H and R555C mutants, R539W is not more but rather less sensitive to PIP₂ decrease than the WT channel. A molecular model of a fragment of the KCNQ1 C-terminus and the membrane bilayer suggested that a potential novel interaction of R539W with cholesterol stabilizes the channel opening and hence prevents rundown upon PIP₂ depletion. We then carried out the same rundown experiments under cholesterol depletion and observed an accelerated R539W rundown that is consistent with this model.

Conclusions

We show for the first time that a mutation may shift the channel interaction with PIP₂ to a preference for cholesterol. This de novo interaction wanes the sensitivity to PIP₂ variations, showing that a mutated channel with a decreased affinity to PIP₂ could paradoxically present a slowed current rundown compared to the WT channel. This suggests that caution is required when using measurements of current rundown as an indicator to compare WT and mutant channel PIP₂ sensitivity.

Heteromerization of PIP aquaporins affects their intrinsic permeability

The plant aquaporin plasma membrane intrinsic proteins (PIP) subfamily represents one of the main gateways for water exchange at the plasma membrane (PM). A fraction of this subfamily, known as PIP1, does not reach the PM unless they are coexpressed with a PIP2 aquaporin. Although ubiquitous and abundantly expressed, the role and properties of PIP1 aquaporins have therefore remained masked. Here, we unravel how FaPIP1;1, a fruit-specific PIP1 aquaporin from Fragaria x ananassa, contributes to the modulation of membrane water permeability (Pf) and pH aquaporin regulation. Our approach was to combine an experimental and mathematical model design to test its activity without affecting its trafficking dynamics. We demonstrate that FaPIP1;1 has a high water channel activity when coexpressed as well as how PIP1-PIP2 affects gating sensitivity in terms of cytosolic acidification. PIP1-PIP2 random heterotetramerization not only allows FaPIP1;1 to arrive at the PM but also produces an enhancement of FaPIP2;1 activity. In this context, we propose that FaPIP1;1 is a key participant in the regulation of water movement across the membranes of cells expressing both aquaporins.

Dynamic subunit stoichiometry confers a progressive continuum of pharmacological sensitivity by KCNQ potassium channels

Voltage-gated KCNQ1 (Kv7.1) potassium channels are expressed abundantly in heart but they are also found in multiple other tissues. Differential coassembly with single transmembrane KCNE beta subunits in different cell types gives rise to a variety of biophysical properties, hence endowing distinct physiological roles for KCNQ1-KCNEx complexes. Mutations in either KCNQ1 or KCNE1 genes result in diseases in brain, heart, and the respiratory system. In addition to complexities arising from existence of five KCNE subunits, KCNE1 to KCNE5, recent studies in heterologous systems suggest unorthodox stoichiometric dynamics in subunit assembly is dependent on KCNE expression levels. The resultant KCNQ1-KCNE channel complexes may have a range of zero to two or even up to four KCNE subunits coassembling per KCNQ1 tetramer. These findings underscore the need to assess the selectivity of small-molecule KCNQ1 modulators on these different assemblies. Here we report a unique small-molecule gating modulator, ML277, that potentiates both homomultimeric KCNQ1 channels and unsaturated heteromultimeric (KCNQ1)4(KCNE1)n (n < 4) channels. Progressive increase of KCNE1 or KCNE3 expression reduces efficacy of ML277 and eventually abolishes ML277-mediated augmentation. In cardiomyocytes, the slowly activating delayed rectifier potassium current, or IKs, is believed to be a heteromultimeric combination of KCNQ1 and KCNE1, but it is not entirely clear whether IKs is mediated by KCNE-saturated KCNQ1 channels or by channels with intermediate stoichiometries. We found ML277 effectively augments IKs current of cultured human cardiomyocytes and shortens action potential duration. These data indicate that unsaturated heteromultimeric (KCNQ1)4(KCNE1)n channels are present as components of IKs and are pharmacologically distinct from KCNE-saturated KCNQ1-KCNE1 channels.

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.

KCNQ1 channels do not undergo concerted but sequential gating transitions in both the absence and the presence of KCNE1 protein

The co-assembly of KCNQ1 with KCNE1 produces I(KS), a K(+) current, crucial for the repolarization of the cardiac action potential. Mutations in these channel subunits lead to life-threatening cardiac arrhythmias. However, very little is known about the gating mechanisms underlying KCNQ1 channel activation. Shaker channels have provided a powerful tool to establish the basic gating mechanisms of voltage-dependent K(+) channels, implying prior independent movement of all four voltage sensor domains (VSDs) followed by channel opening via a last concerted cooperative transition. To determine the nature of KCNQ1 channel gating, we performed a thermodynamic mutant cycle analysis by constructing a concatenated tetrameric KCNQ1 channel and by introducing separately a gain and a loss of function mutation, R231W and R243W, respectively, into the S4 helix of the VSD of one, two, three, and four subunits. The R231W mutation destabilizes channel closure and produces constitutively open channels, whereas the R243W mutation disrupts channel opening solely in the presence of KCNE1 by right-shifting the voltage dependence of activation. The linearity of the relationship between the shift in the voltage dependence of activation and the number of mutated subunits points to an independence of VSD movements, with each subunit incrementally contributing to channel gating. Contrary to Shaker channels, our work indicates that KCNQ1 channels do not experience a late cooperative concerted opening transition. Our data suggest that KCNQ1 channels in both the absence and the presence of KCNE1 undergo sequential gating transitions leading to channel opening even before all VSDs have moved.

The KCNE Tango - How KCNE1 Interacts with Kv7.1

The classical tango is a dance characterized by a 2/4 or 4/4 rhythm in which the partners dance in a coordinated way, allowing dynamic contact. There is a surprising similarity between the tango and how KCNE β-subunits "dance" to the fast rhythm of the cell with their partners from the Kv channel family. The five KCNE β-subunits interact with several members of the Kv channels, thereby modifying channel gating via the interaction of their single transmembrane-spanning segment, the extracellular amino terminus, and/or the intracellular carboxy terminus with the Kv α-subunit. Best studied is the molecular basis of interactions between KCNE1 and Kv7.1, which, together, supposedly form the native cardiac I(Ks) channel. Here we review the current knowledge about functional and molecular interactions of KCNE1 with Kv7.1 and try to summarize and interpret the tango of the KCNEs.

Stoichiometry of the slow I(ks) potassium channel in human embryonic stem cell-derived myocytes

The delayed rectifier I(ks) potassium channel is composed of α-(KCNQ1) and β-(KCNE1) subunits. The stoichiometry of I(ks) channels is a matter of some debate. Recently some investigators proposed that the number of KCNE1 subunits per KCNQ1 tetramer could be vary from one to four depending on the relative expression of these two genes. Here we review our previous study of biophysical properties of I(ks) in human embryonic stem cell-derived cardiomyocytes (hESC-CMs) showed that I(ks) in hESC-CMs is a coassembly channel with a stoichiometry other than 1:1, which could be further modulated by additional KCNE1.

The A-kinase anchoring protein Yotiao facilitates complex formation between adenylyl cyclase type 9 and the IKs potassium channel in heart

The scaffolding protein Yotiao is a member of a large family of protein A-kinase anchoring proteins with important roles in the organization of spatial and temporal signaling. In heart, Yotiao directly associates with the slow outward potassium ion current (I(Ks)) and recruits both PKA and PP1 to regulate I(Ks) phosphorylation and gating. Human mutations that disrupt I(Ks)-Yotiao interaction result in reduced PKA-dependent phosphorylation of the I(Ks) subunit KCNQ1 and inhibition of sympathetic stimulation of I(Ks), which can give rise to long-QT syndrome. We have previously identified a subset of adenylyl cyclase (AC) isoforms that interact with Yotiao, including AC1-3 and AC9, but surprisingly, this group did not include the major cardiac isoforms AC5 and AC6. We now show that either AC2 or AC9 can associate with KCNQ1 in a complex mediated by Yotiao. In transgenic mouse heart expressing KCNQ1-KCNE1, AC activity was specifically associated with the I(Ks)-Yotiao complex and could be disrupted by addition of the AC9 N terminus. A survey of all AC isoforms by RT-PCR indicated expression of AC4-6 and AC9 in adult mouse cardiac myocytes. Of these, the only Yotiao-interacting isoform was AC9. Furthermore, the endogenous I(Ks)-Yotiao complex from guinea pig also contained AC9. Finally, AC9 association with the KCNQ1-Yotiao complex sensitized PKA phosphorylation of KCNQ1 to β-adrenergic stimulation. Thus, in heart, Yotiao brings together PKA, PP1, PDE4D3, AC9, and the I(Ks) channel to achieve localized temporal regulation of β-adrenergic stimulation.

How does KCNE1 regulate the Kv7.1 potassium channel? Model-structure, mutations, and dynamics of the Kv7.1-KCNE1 complex

The voltage-gated potassium channel Kv7.1 and its auxiliary subunit KCNE1 are expressed in the heart and give rise to the major repolarization current. The interaction of Kv7.1 with the single transmembrane helix of KCNE1 considerably slows channel activation and deactivation, raises single-channel conductance, and prevents slow voltage-dependent inactivation. We built a Kv7.1-KCNE1 model-structure. The model-structure agrees with previous disulfide mapping studies and enables us to derive molecular interpretations of electrophysiological recordings that we obtained for two KCNE1 mutations. An elastic network analysis of Kv7.1 fluctuations in the presence and absence of KCNE1 suggests a mechanistic perspective on the known effects of KCNE1 on Kv7.1 function: slow deactivation is attributed to the low mobility of the voltage-sensor domains upon KCNE1 binding, abolishment of voltage-dependent inactivation could result from decreased fluctuations in the external vestibule, and amalgamation of the fluctuations in the pore region is associated with enhanced ion conductivity.

Opposite Effects of the S4-S5 Linker and PIP(2) on Voltage-Gated Channel Function: KCNQ1/KCNE1 and Other Channels

Voltage-gated potassium (Kv) channels are tetramers, each subunit presenting six transmembrane segments (S1-S6), with each S1-S4 segments forming a voltage-sensing domain (VSD) and the four S5-S6 forming both the conduction pathway and its gate. S4 segments control the opening of the intracellular activation gate in response to changes in membrane potential. Crystal structures of several voltage-gated ion channels in combination with biophysical and mutagenesis studies highlighted the critical role of the S4-S5 linker (S4S5(L)) and of the S6 C-terminal part (S6(T)) in the coupling between the VSD and the activation gate. Several mechanisms have been proposed to describe the coupling at a molecular scale. This review summarizes the mechanisms suggested for various voltage-gated ion channels, including a mechanism that we described for KCNQ1, in which S4S5(L) is acting like a ligand binding to S6(T) to stabilize the channel in a closed state. As discussed in this review, this mechanism may explain the reverse response to depolarization in HCN-like channels. As opposed to S4S5(L), the phosphoinositide, phosphatidylinositol 4,5-bisphosphate (PIP(2)), stabilizes KCNQ1 channel in an open state. Many other ion channels (not only voltage-gated) require PIP(2) to function properly, confirming its crucial importance as an ion channel cofactor. This is highlighted in cases in which an altered regulation of ion channels by PIP(2) leads to channelopathies, as observed for KCNQ1. This review summarizes the state of the art on the two regulatory mechanisms that are critical for KCNQ1 and other voltage-gated channels function (PIP(2) and S4S5(L)), and assesses their potential physiological and pathophysiological roles.

KCNQ1 subdomains involved in KCNE modulation revealed by an invertebrate KCNQ1 orthologue

KCNQ1 channels are voltage-gated potassium channels that are widely expressed in various non-neuronal tissues, such as the heart, pancreas, and intestine. KCNE proteins are known as the auxiliary subunits for KCNQ1 channels. The effects and functions of the different KCNE proteins on KCNQ1 modulation are various; the KCNQ1–KCNE1 ion channel complex produces a slowly activating potassium channel that is crucial for heartbeat regulation, while the KCNE3 protein makes KCNQ1 channels constitutively active, which is important for K⁺ and Cl⁻ transport in the intestine. The mechanisms by which KCNE proteins modulate KCNQ1 channels have long been studied and discussed; however, it is not well understood how different KCNE proteins exert considerably different effects on KCNQ1 channels. Here, we approached this point by taking advantage of the recently isolated Ci-KCNQ1, a KCNQ1 homologue from marine invertebrate Ciona intestinalis. We found that Ci-KCNQ1 alone could be expressed in Xenopus laevis oocytes and produced a voltage-dependent potassium current, but that Ci-KCNQ1 was not properly modulated by KCNE1 and totally unaffected by coexpression of KCNE3. By making chimeras of Ci-KCNQ1 and human KCNQ1, we determined several amino acid residues located in the pore region of human KCNQ1 involved in KCNE1 modulation. Interestingly, though, these amino acid residues of the pore region are not important for KCNE3 modulation, and we subsequently found that the S1 segment plays an important role in making KCNQ1 channels constitutively active by KCNE3. Our findings indicate that different KCNE proteins use different domains of KCNQ1 channels, and that may explain why different KCNE proteins give quite different outcomes by forming a complex with KCNQ1 channels.

Characterization of KCNQ1 atrial fibrillation mutations reveals distinct dependence on KCNE1

The I(Ks) potassium channel, critical to control of heart electrical activity, requires assembly of α (KCNQ1) and β (KCNE1) subunits. Inherited mutations in either I(Ks) channel subunit are associated with cardiac arrhythmia syndromes. Two mutations (S140G and V141M) that cause familial atrial fibrillation (AF) are located on adjacent residues in the first membrane-spanning domain of KCNQ1, S1. These mutations impair the deactivation process, causing channels to appear constitutively open. Previous studies suggest that both mutant phenotypes require the presence of KCNE1. Here we found that despite the proximity of these two mutations in the primary protein structure, they display different functional dependence in the presence of KCNE1. In the absence of KCNE1, the S140G mutation, but not V141M, confers a pronounced slowing of channel deactivation and a hyperpolarizing shift in voltage-dependent activation. When coexpressed with KCNE1, both mutants deactivate significantly slower than wild-type KCNQ1/KCNE1 channels. The differential dependence on KCNE1 can be correlated with the physical proximity between these positions and KCNE1 as shown by disulfide cross-linking studies: V141C forms disulfide bonds with cysteine-substituted KCNE1 residues, whereas S140C does not. These results further our understanding of the structural relationship between KCNE1 and KCNQ1 subunits in the I(Ks) channel, and provide mechanisms for understanding the effects on channel deactivation underlying these two atrial fibrillation mutations.

KCNE2 and the K (+) channel: the tail wagging the dog

KCNE2, originally designated MinK-related peptide 1 (MiRP1), belongs to a five-strong family of potassium channel ancillary (β) subunits that, despite the diminutive size of the family and its members, has loomed large in the field of ion channel physiology. KCNE2 dictates K (+) channel gating, conductance, α subunit composition, trafficking and pharmacology, and also modifies functional properties of monovalent cation-nonselective HCN channels. The Kcne2 (-/-) mouse exhibits cardiac arrhythmia and hypertrophy, achlorhydria, gastric neoplasia, hypothyroidism, alopecia, stunted growth and choroid plexus epithelial dysfunction, illustrating the breadth and depth of the influence of KCNE2, mutations which are also associated with human cardiac arrhythmias. Here, the modus operandi and physiological roles of this potent regulator of membrane excitability and ion secretion are reviewed with particular emphasis on the ability of KCNE2 to shape the electrophysiological landscape of both excitable and non-excitable cells.

Biophysical properties of slow potassium channels in human embryonic stem cell derived cardiomyocytes implicate subunit stoichiometry

Human embryonic stem cells (hESCs) are an important cellular model for studying ion channel function in the context of a human cardiac cell and will provide a wealth of information about both heritable arrhythmias and acquired electrophysiological disorders. However, detailed electrophysiological characterization of the important cardiac ion channels has been so far overlooked. Because mutations in the gene for the I(Ks) α subunit, KCNQ1, constitute the majority of long QT syndrome (LQT-1) cases, we have carried out a detailed biophysical analysis of this channel expressed in hESCs to establish baseline I(Ks) channel biophysical properties in cardiac myocytes derived from hESCs (hESC-CMs). I(Ks) channels are heteromultimeric proteins consisting of four identical α-subunits (KCNQ1) assembled with auxiliary β-subunits (KCNE1). We found that the half-maximal I(Ks) activation voltage in hESC-CMs and in myocytes derived from human induced pluripotent stems cells (hiPSC-CMs) falls between that of KCNQ1 channels expressed alone and with full complement of KCNE1, the major KCNE subunit expressed in hESC-CMs as shown by qPCR analysis. Overexpression of KCNE1 by transfection of hESC-CMs markedly shifted and slowed native I(Ks) activation implying assembly of additional KCNE1 subunits with endogenous channels. Our results in hESC-CMs, which indicate an I(Ks) subunit stoichiometry that can be altered by variable KCNE1 expression, suggest the possibility for variable I(Ks) function in the developing heart, in different tissues in the heart, and in disease. This establishes a new baseline for I(Ks) channel properties in myocytes derived from pluripotent stem cells and will guide future studies in patient-specific hiPSCs.

Partial restoration of the long QT syndrome associated KCNQ1 A341V mutant by the KCNE1 β-subunit

BACKGROUND

The A341V mutation in the pore-forming KCNQ1 subunit of the slowly activating delayed-rectifier potassium current (IKs) underlies a common form of the long QT syndrome, and is associated with an unusually severe phenotype. However, there is controversy regarding the underlying mechanism responsible for the clinically observed phenotype. We investigated the biophysical characteristics of A341V in a cardiac environment by utilizing a cardiac cell line, and in particular the impact of the KCNE1 β-subunit.

METHODS

Whole-cell current were recorded from transiently transfected HL-1 cells, a cardiac cell line. Mutant KCNQ1 and KCNE1 were constructed by site-directed mutagenesis.

RESULTS

The A341V mutant resulted in a non-functional channel when expressed alone. When co-expressed with wild type KCNE1, A341V produced a slowly activating current, with a smaller current density, slower rates of activation, and a depolarized shift in its activation curve compared to the wild type KCNQ1+KCNE1. Confocal microscopy confirmed the surface expression of GFP-tagged A341V, suggesting a functionally defective protein. A T58A mutation in KCNE1 abolished functional restoration of A341V. Under heterozygous conditions, the expression of A341V+KCNQ1+KCNE1 reduced but did not abolish the electrophysiological changes observed in A341V+KCNE1. A dominant negative effect of A341V was also observed. Action potential simulations revealed that the A341V mutation is arrhythmogenic.

CONCLUSIONS

The KCNE1 β-subunit partially rescued the non-functional A341V mutant, with electrophysiological properties distinct from the wild type IKs.

GENERAL SIGNIFICANCE

The severity of the A341V phenotype may be due to a combination of a significant suppression of the IKs with altered biophysical characteristics.

Structural basis of slow activation gating in the cardiac I Ks channel complex

Accessory β-subunits of the KCNE gene family modulate the function of various cation channel α-subunits by the formation of heteromultimers. Among the most dramatic changes of biophysical properties of a voltage-gated channel by KCNEs are the effects of KCNE1 on KCNQ1 channels. KCNQ1 and KCNE1 are believed to form nativeI(Ks) channels. Here, we characterize molecular determinants of KCNE1 interaction with KCNQ1 channels by scanning mutagenesis, double mutant cycle analysis, and molecular dynamics simulations. Our findings suggest that KCNE1 binds to the outer face of the KCNQ1 channel pore domain, modifies interactions between voltage sensor, S4-S5 linker and the pore domain, leading to structural modifications of the selectivity filter and voltage sensor domain. Molecular dynamics simulations suggest a stable interaction of the KCNE1 transmembrane α-helix with the pore domain S5/S6 and part of the voltage sensor domain S4 of KCNQ1 in a putative pre-open channel state. Formation of this state may induce slow activation gating, the pivotal characteristic of native cardiac I(Ks) channels. This new KCNQ1-KCNE1 model may become useful for dynamic modeling of disease-associated mutant I(Ks) channels.

O-glycosylation of the cardiac I(Ks) complex

Post-translational modifications of the KCNQ1–KCNE1 (Kv7) K+ channel complex are vital for regulation of the cardiac IKs current and action potential duration. Here, we show the KCNE1 regulatory subunit is O-glycosylated with mucin-type glycans in vivo. As O-linked glycosylation sites are not recognizable by sequence gazing, we designed a novel set of glycosylation mutants and KCNE chimeras and analysed their glycan content using deglycosylation enzymes. Our results show that KCNE1 is exclusively O-glycosylated at Thr-7, which is also required for N-glycosylation at Asn-5. For wild type KCNE1, the overlapping N- and O-glycosylation sites are innocuous for subunit biogenesis; however, mutation of Thr-7 to a non-hydroxylated residue yielded mostly unglycosylated protein and a small fraction of mono-N-glycosylated protein. The compounded hypoglycosylation was equally deleterious for KCNQ1–KCNE1 cell surface expression, demonstrating that KCNE1 O-glycosylation is a post-translational modification that is integral for the proper biogenesis and anterograde trafficking of the cardiac IKs complex. The enzymatic assays and panel of glycosylation mutants used here will be valuable for identifying the different KCNE1 glycoforms in native cells and determining the roles N- and O-glycosylation play in KCNQ1–KCNE1 function and localization in cardiomyocytes,

KCNE1 enhances phosphatidylinositol 4,5-bisphosphate (PIP2) sensitivity of IKs to modulate channel activity

Phosphatidylinositol 4,5-bisphosphate (PIP(2)) is necessary for the function of various ion channels. The potassium channel, I(Ks), is important for cardiac repolarization and requires PIP(2) to activate. Here we show that the auxiliary subunit of I(Ks), KCNE1, increases PIP(2) sensitivity 100-fold over channels formed by the pore-forming KCNQ1 subunits alone, which effectively amplifies current because native PIP(2) levels in the membrane are insufficient to activate all KCNQ1 channels. A juxtamembranous site in the KCNE1 C terminus is a key structural determinant of PIP(2) sensitivity. Long QT syndrome associated mutations of this site lower PIP(2) affinity, resulting in reduced current. Application of exogenous PIP(2) to these mutants restores wild-type channel activity. These results reveal a vital role of PIP(2) for KCNE1 modulation of I(Ks) channels that may represent a common mechanism of auxiliary subunit modulation of many ion channels.