Human iPSC-derived cardiomyocytes and pyridyl-phenyl mexiletine analogs.

In the United States, approximately one million individuals are hospitalized every year for arrhythmias, making arrhythmias one of the top causes of healthcare expenditures. Mexiletine is currently used as an antiarrhythmic drug but has limitations. The purpose of this work was to use normal and Long QT syndrome Type 3 (LQTS3) patient-derived human induced pluripotent stem cell (iPSC)-derived cardiomyocytes to identify an analog of mexiletine with superior drug-like properties. Compared to racemic mexiletine, medicinal chemistry optimization of substituted racemic pyridyl phenyl mexiletine analogs resulted in a more potent sodium channel inhibitor with greater selectivity for the sodium over the potassium channel and for late over peak sodium current.

KCNE1 and KCNE3 modulate KCNQ1 channels by affecting different gating transitions.

KCNE ß-subunits assemble with and modulate the properties of voltage-gated K+ channels. In the heart, KCNE1 associates with the α-subunit KCNQ1 to generate the slowly activating, voltage-dependent potassium current (IKs) in the heart that controls the repolarization phase of cardiac action potentials. By contrast, in epithelial cells from the colon, stomach, and kidney, KCNE3 coassembles with KCNQ1 to form K+ channels that are voltage-independent K+ channels in the physiological voltage range and important for controlling water and salt secretion and absorption. How KCNE1 and KCNE3 subunits modify KCNQ1 channel gating so differently is largely unknown. Here, we use voltage clamp fluorometry to determine how KCNE1 and KCNE3 affect the voltage sensor and the gate of KCNQ1. By separating S4 movement and gate opening by mutations or phosphatidylinositol 4,5-bisphosphate depletion, we show that KCNE1 affects both the S4 movement and the gate, whereas KCNE3 affects the S4 movement and only affects the gate in KCNQ1 if an intact S4-to-gate coupling is present. Further, we show that a triple mutation in the middle of the transmembrane (TM) segment of KCNE3 introduces KCNE1-like effects on the second S4 movement and the gate. In addition, we show that differences in two residues at the external end of the KCNE TM segments underlie differences in the effects of the different KCNEs on the first S4 movement and the voltage sensor-to-gate coupling.

Detection of Nav1.5 Conformational Change in Mammalian Cells Using the Noncanonical Amino Acid ANAP.

Nav1.5 inactivation is necessary for healthy conduction of the cardiac action potential. Genetic mutations of Nav1.5 perturb inactivation and cause potentially fatal arrhythmias associated with long QT syndrome type 3. The exact structural dynamics of the inactivation complex is unknown. To sense inactivation gate conformational change in live mammalian cells, we incorporated the solvatochromic fluorescent noncanonical amino acid 3-((6-acetylnaphthalen-2-yl)amino)-2-aminopropanoic acid (ANAP) into single sites in the Nav1.5 inactivation gate. ANAP was incorporated in full-length and C-terminally truncated Nav1.5 channels using mammalian cell synthetase-tRNA technology. ANAP-incorporated channels were expressed in mammalian cells, and they exhibited pathophysiological function. A spectral imaging potassium depolarization assay was designed to detect ANAP emission shifts associated with Nav1.5 conformational change. Site-specific intracellular ANAP incorporation affords live-cell imaging and detection of Nav1.5 inactivation gate conformational change in mammalian cells.

A novel channelopathy in pulmonary arterial hypertension.

BACKGROUND: Pulmonary arterial hypertension is a devastating disease with high mortality. Familial cases of pulmonary arterial hypertension are usually characterized by autosomal dominant transmission with reduced penetrance, and some familial cases have unknown genetic causes. METHODS: We studied a family in which multiple members had pulmonary arterial hypertension without identifiable mutations in any of the genes known to be associated with the disease, including BMPR2, ALK1, ENG, SMAD9, and CAV1. Three family members were studied with whole-exome sequencing. Additional patients with familial or idiopathic pulmonary arterial hypertension were screened for the mutations in the gene that was identified on whole-exome sequencing. All variants were expressed in COS-7 cells, and channel function was studied by means of patch-clamp analysis. RESULTS: We identified a novel heterozygous missense variant c.608 G→A (G203D) in KCNK3 (the gene encoding potassium channel subfamily K, member 3) as a disease-causing candidate gene in the family. Five additional heterozygous missense variants in KCNK3 were independently identified in 92 unrelated patients with familial pulmonary arterial hypertension and 230 patients with idiopathic pulmonary arterial hypertension. We used in silico bioinformatic tools to predict that all six novel variants would be damaging. Electrophysiological studies of the channel indicated that all these missense mutations resulted in loss of function, and the reduction in the potassium-channel current was remedied by the application of the phospholipase inhibitor ONO-RS-082. CONCLUSIONS: Our study identified the association of a novel gene, KCNK3, with familial and idiopathic pulmonary arterial hypertension. Mutations in this gene produced reduced potassium-channel current, which was successfully remedied by pharmacologic manipulation. (Funded by the National Institutes of Health.)

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.

Deranged sodium to sudden death.

In February 2014, a group of scientists convened as part of the University of California Davis Cardiovascular Symposium to bring together experimental and mathematical modelling perspectives and discuss points of consensus and controversy on the topic of sodium in the heart. This paper summarizes the topics of presentation and discussion from the symposium, with a focus on the role of aberrant sodium channels and abnormal sodium homeostasis in cardiac arrhythmias and pharmacotherapy from the subcellular scale to the whole heart. Two following papers focus on Na(+) channel structure, function and regulation, and Na(+)/Ca(2+) exchange and Na(+)/K(+) ATPase. The UC Davis Cardiovascular Symposium is a biannual event that aims to bring together leading experts in subfields of cardiovascular biomedicine to focus on topics of importance to the field. The focus on Na(+) in the 2014 symposium stemmed from the multitude of recent studies that point to the importance of maintaining Na(+) homeostasis in the heart, as disruption of homeostatic processes are increasingly identified in cardiac disease states. Understanding how disruption in cardiac Na(+)-based processes leads to derangement in multiple cardiac components at the level of the cell and to then connect these perturbations to emergent behaviour in the heart to cause disease is a critical area of research. The ubiquity of disruption of Na(+) channels and Na(+) homeostasis in cardiac disorders of excitability and mechanics emphasizes the importance of a fundamental understanding of the associated mechanisms and disease processes to ultimately reveal new targets for human therapy.

Ranolazine for congenital and acquired late INa-linked arrhythmias: in silico pharmacological screening.

RATIONALE: The antianginal ranolazine blocks the human ether-a-go-go-related gene-based current IKr at therapeutic concentrations and causes QT interval prolongation. Thus, ranolazine is contraindicated for patients with preexisting long-QT and those with repolarization abnormalities. However, with its preferential targeting of late INa (INaL), patients with disease resulting from increased INaL from inherited defects (eg, long-QT syndrome type 3 or disease-induced electric remodeling (eg, ischemic heart failure) might be exactly the ones to benefit most from the presumed antiarrhythmic properties of ranolazine. OBJECTIVE: We developed a computational model to predict if therapeutic effects of pharmacological targeting of INaL by ranolazine prevailed over the off-target block of IKr in the setting of inherited long-QT syndrome type 3 and heart failure. METHODS AND RESULTS: We developed computational models describing the kinetics and the interaction of ranolazine with cardiac Na(+) channels in the setting of normal physiology, long-QT syndrome type 3-linked ΔKPQ mutation, and heart failure. We then simulated clinically relevant concentrations of ranolazine and predicted the combined effects of Na(+) channel and IKr blockade by both the parent compound ranolazine and its active metabolites, which have shown potent blocking effects in the therapeutically relevant range. Our simulations suggest that ranolazine is effective at normalizing arrhythmia triggers in bradycardia-dependent arrhythmias in long-QT syndrome type 3 as well tachyarrhythmogenic triggers arising from heart failure-induced remodeling. CONCLUSIONS: Our model predictions suggest that acute targeting of INaL with ranolazine may be an effective therapeutic strategy in diverse arrhythmia-provoking situations that arise from a common pathway of increased pathological INaL.

Unique cardiac Purkinje fiber transient outward current ß-subunit composition: a potential molecular link to idiopathic ventricular fibrillation.

RATIONALE: A chromosomal haplotype producing cardiac overexpression of dipeptidyl peptidase-like protein-6 (DPP6) causes familial idiopathic ventricular fibrillation. The molecular basis of transient outward current (I(to)) in Purkinje fibers (PFs) is poorly understood. We hypothesized that DPP6 contributes to PF I(to) and that its overexpression might specifically alter PF I(to) properties and repolarization. OBJECTIVE: To assess the potential role of DPP6 in PF I(to). METHODS AND RESULTS: Clinical data in 5 idiopathic ventricular fibrillation patients suggested arrhythmia origin in the PF-conducting system. PF and ventricular muscle I(to) had similar density, but PF I(to) differed from ventricular muscle in having tetraethylammonium sensitivity and slower recovery. DPP6 overexpression significantly increased, whereas DPP6 knockdown reduced, I(to) density and tetraethylammonium sensitivity in canine PF but not in ventricular muscle cells. The K(+)-channel interacting ß-subunit K(+)-channel interacting protein type-2, essential for normal expression of I(to) in ventricular muscle, was weakly expressed in human PFs, whereas DPP6 and frequenin (neuronal calcium sensor-1) were enriched. Heterologous expression of Kv4.3 in Chinese hamster ovary cells produced small I(to); I(to) amplitude was greatly enhanced by coexpression with K(+)-channel interacting protein type-2 or DPP6. Coexpression of DPP6 with Kv4.3 and K(+)-channel interacting protein type-2 failed to alter I(to) compared with Kv4.3/K(+)-channel interacting protein type-2 alone, but DPP6 expression with Kv4.3 and neuronal calcium sensor-1 (to mimic PF I(to) composition) greatly enhanced I(to) compared with Kv4.3/neuronal calcium sensor-1 and recapitulated characteristic PF kinetic/pharmacological properties. A mathematical model of cardiac PF action potentials showed that I(to) enhancement can greatly accelerate PF repolarization. CONCLUSIONS: These results point to a previously unknown central role of DPP6 in PF I(to), with DPP6 gain of function selectively enhancing PF current, and suggest that a DPP6-mediated PF early-repolarization syndrome might be a novel molecular paradigm for some forms of idiopathic ventricular fibrillation.

Allosteric gating mechanism underlies the flexible gating of KCNQ1 potassium channels.

KCNQ1 (Kv7.1) is a unique member of the superfamily of voltage-gated K(+) channels in that it displays a remarkable range of gating behaviors tuned by coassembly with different ß subunits of the KCNE family of proteins. To better understand the basis for the biophysical diversity of KCNQ1 channels, we here investigate the basis of KCNQ1 gating in the absence of ß subunits using voltage-clamp fluorometry (VCF). In our previous study, we found the kinetics and voltage dependence of voltage-sensor movements are very similar to those of the channel gate, as if multiple voltage-sensor movements are not required to precede gate opening. Here, we have tested two different hypotheses to explain KCNQ1 gating: (i) KCNQ1 voltage sensors undergo a single concerted movement that leads to channel opening, or (ii) individual voltage-sensor movements lead to channel opening before all voltage sensors have moved. Here, we find that KCNQ1 voltage sensors move relatively independently, but that the channel can conduct before all voltage sensors have activated. We explore a KCNQ1 point mutation that causes some channels to transition to the open state even in the absence of voltage-sensor movement. To interpret these results, we adopt an allosteric gating scheme wherein KCNQ1 is able to transition to the open state after zero to four voltage-sensor movements. This model allows for widely varying gating behavior, depending on the relative strength of the opening transition, and suggests how KCNQ1 could be controlled by coassembly with different KCNE family members.

Decoding polyubiquitin regulation of KV7. 1 functional expression with engineered linkage-selective deubiquitinases.

Protein posttranslational modification with distinct polyubiquitin linkage chains is a critical component of the 'ubiquitin code' that universally regulates protein expression and function to control biology. Functional consequences of diverse polyubiquitin linkages on proteins are mostly unknown, with progress hindered by a lack of methods to specifically tune polyubiquitin linkages on individual proteins in live cells. Here, we bridge this gap by exploiting deubiquitinases (DUBs) with preferences for hydrolyzing different polyubiquitin linkages: OTUD1 - K63; OTUD4 - K48; Cezanne - K11; TRABID - K29/K33; and USP21 - non-specific. We developed a suite of engineered deubiquitinases (enDUBs) comprised of DUB catalytic domains fused to a GFP-targeted nanobody and used them to investigate polyubiquitin linkage regulation of an ion channel, YFP-KCNQ1. Mass spectrometry of YFP-KCNQ1 expressed in HEK293 cells indicated channel polyubiquitination with K48 (72%) and K63 (24%) linkages being dominant. NEDD4-2 and ITCH both decreased KCNQ1 functional expression but with distinctive polyubiquitination signatures. All enDUBs reduced KCNQ1 ubiquitination but yielded unique effects on channel expression, surface density, ionic currents, and subcellular localization. The pattern of outcomes indicates K11, K29/K33, and K63 chains mediate net KCNQ1-YFP intracellular retention, but achieved in different ways: K11 promotes ER retention/degradation, enhances endocytosis, and reduces recycling; K29/K33 promotes ER retention/degradation; K63 enhances endocytosis and reduces recycling. The pattern of enDUB effects on KCNQ1-YFP differed in cardiomyocytes, emphasizing ubiquitin code mutability. Surprisingly, enDUB-O4 decreased KCNQ1-YFP surface density suggesting a role for K48 in forward trafficking. Lastly, linkage-selective enDUBs displayed varying capabilities to rescue distinct trafficking-deficient long QT syndrome type 1 mutations. The results reveal distinct polyubiquitin chains control different aspects of KCNQ1 functional expression, demonstrate ubiquitin code plasticity, and introduce linkage-selective enDUBs as a potent tool to help demystify the polyubiquitin code.

Ion channel inhibition by targeted recruitment of NEDD4-2 with divalent nanobodies.

Targeted recruitment of E3 ubiquitin ligases to degrade traditionally undruggable proteins is a disruptive paradigm for developing new therapeutics. Two salient limitations are that <2% of the ~600 E3 ligases in the human genome have been exploited to produce proteolysis targeting chimeras (PROTACs), and the efficacy of the approach has not been demonstrated for a vital class of complex multi-subunit membrane proteins- ion channels. NEDD4-1 and NEDD4-2 are physiological regulators of myriad ion channels, and belong to the 28-member HECT (homologous to E6AP C-terminus) family of E3 ligases with widespread roles in cell/developmental biology and diverse diseases including various cancers, immunological and neurological disorders, and chronic pain. The potential efficacy of HECT E3 ligases for targeted protein degradation is unexplored, constrained by a lack of appropriate binders, and uncertain due to their complex regulation by layered intra-molecular and posttranslational mechanisms. Here, we identified a nanobody that binds with high affinity and specificity to a unique site on the N-lobe of the NEDD4-2 HECT domain at a location physically separate from sites critical for catalysis- the E2 binding site, the catalytic cysteine, and the ubiquitin exosite- as revealed by a 3.1 Å cryo-electron microscopy reconstruction. Recruiting endogenous NEDD4-2 to diverse ion channel proteins (KCNQ1, ENaC, and CaV2.2) using a divalent (DiVa) nanobody format strongly reduced their functional expression with minimal off-target effects as assessed by global proteomics, compared to simple NEDD4-2 overexpression. The results establish utility of a HECT E3 ligase for targeted protein downregulation, validate a class of complex multi-subunit membrane proteins as susceptible to this modality, and introduce endogenous E3 ligase recruitment with DiVa nanobodies as a general method to generate novel genetically-encoded ion channel inhibitors.

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 ß-adrenergic stimulation. Thus, in heart, Yotiao brings together PKA, PP1, PDE4D3, AC9, and the I(Ks) channel to achieve localized temporal regulation of ß-adrenergic stimulation.

KCNE1 alters the voltage sensor movements necessary to open the KCNQ1 channel gate.

The delayed rectifier I(Ks) potassium channel, formed by coassembly of α- (KCNQ1) and ß- (KCNE1) subunits, is essential for cardiac function. Although KCNE1 is necessary to reproduce the functional properties of the native I(Ks) channel, the mechanism(s) through which KCNE1 modulates KCNQ1 is unknown. Here we report measurements of voltage sensor movements in KCNQ1 and KCNQ1/KCNE1 channels using voltage clamp fluorometry. KCNQ1 channels exhibit indistinguishable voltage dependence of fluorescence and current signals, suggesting a one-to-one relationship between voltage sensor movement and channel opening. KCNE1 coexpression dramatically separates the voltage dependence of KCNQ1/KCNE1 current and fluorescence, suggesting an imposed requirement for movements of multiple voltage sensors before KCNQ1/KCNE1 channel opening. This work provides insight into the mechanism by which KCNE1 modulates the I(Ks) channel and presents a mechanism for distinct ß-subunit regulation of ion channel proteins.

Divergent regulation of KCNQ1/E1 by targeted recruitment of protein kinase A to distinct sites on the channel complex.

The slow delayed rectifier potassium current, IKs, conducted through pore-forming Q1 and auxiliary E1 ion channel complexes is important for human cardiac action potential repolarization. During exercise or fright, IKs is up-regulated by protein kinase A (PKA)-mediated Q1 phosphorylation to maintain heart rhythm and optimum cardiac performance. Sympathetic up-regulation of IKs requires recruitment of PKA holoenzyme (two regulatory - RI or RII - and two catalytic Cα subunits) to Q1 C-terminus by an A kinase anchoring protein (AKAP9). Mutations in Q1 or AKAP9 that abolish their functional interaction result in long QT syndrome type 1 and 11, respectively, which increases the risk of sudden cardiac death during exercise. Here, we investigated the utility of a targeted protein phosphorylation (TPP) approach to reconstitute PKA regulation of IKs in the absence of AKAP9. Targeted recruitment of endogenous Cα to E1-YFP using a GFP/YFP nanobody (nano) fused to RIIα enabled acute cAMP-mediated enhancement of IKs, reconstituting physiological regulation of the channel complex. By contrast, nano-mediated tethering of RIIα or Cα to Q1-YFP constitutively inhibited IKs by retaining the channel intracellularly in the endoplasmic reticulum and Golgi. Proteomic analysis revealed that distinct phosphorylation sites are modified by Cα targeted to Q1-YFP compared to free Cα. Thus, functional outcomes of synthetically recruited PKA on IKs regulation is critically dependent on the site of recruitment within the channel complex. The results reveal insights into divergent regulation of IKs by phosphorylation across different spatial and time scales, and suggest a TPP approach to develop new drugs to prevent exercise-induced sudden cardiac death.

Location of KCNE1 relative to KCNQ1 in the I(KS) potassium channel by disulfide cross-linking of substituted cysteines.

The cardiac-delayed rectifier K(+) current (I(KS)) is carried by a complex of KCNQ1 (Q1) subunits, containing the voltage-sensor domains and the pore, and auxiliary KCNE1 (E1) subunits, required for the characteristic I(KS) voltage dependence and kinetics. To locate the transmembrane helix of E1 (E1-TM) relative to the Q1 TM helices (S1-S6), we mutated, one at a time, the first four residues flanking the extracellular ends of S1-S6 and E1-TM to Cys, coexpressed all combinations of Q1 and E1 Cys-substituted mutants in CHO cells, and determined the extents of spontaneous disulfide-bond formation. Cys-flanking E1-TM readily formed disulfides with Cys-flanking S1 and S6, much less so with the S3-S4 linker, and not at all with S2 or S5. These results imply that the extracellular flank of the E1-TM is located between S1 and S6 on different subunits of Q1. The salient functional effects of selected cross-links were as follows. A disulfide from E1 K41C to S1 I145C strongly slowed deactivation, and one from E1 L42C to S6 V324C eliminated deactivation. Given that E1-TM is between S1 and S6 and that K41C and L42C are likely to point approximately oppositely, these two cross-links are likely to favor similar axial rotations of E1-TM. In the opposite orientation, a disulfide from E1 K41C to S6 V324C slightly slowed activation, and one from E1 L42C to S1 I145C slightly speeded deactivation. Thus, the first E1 orientation strongly favors the open state, while the approximately opposite orientation favors the closed state.

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 ß-(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.

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 IKS 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 IKS 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 IKS 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 IKS complexes-KCNE1-G52R and KCNE1-L51H and in heterogeneous IKS channel complexes which mimic heterogeneous expression of mutations in patients. ML277 and R-L3 increased the mutant IKS current amplitude and slowed current deactivation, but not in wild type (WT) IKS. 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.