Homozygous missense N629D hERG (KCNH2) potassium channel mutation causes developmental defects in the right ventricle and its outflow tract and embryonic lethality

Levels of Small Extracellular Vesicles Containing hERG-1 and Hsp47 as Potential Biomarkers for Cardiovascular Diseases

The diagnosis of cardiovascular disease (CVD) is still limited. Therefore, this study demonstrates the presence of human ether-a-go-go-related gene 1 (hERG1) and heat shock protein 47 (Hsp47) on the surface of small extracellular vesicles (sEVs) in human peripheral blood and their association with CVD. In this research, 20 individuals with heart failure and 26 participants subjected to cardiac stress tests were enrolled. The associations between hERG1 and/or Hsp47 in sEVs and CVD were established using Western blot, flow cytometry, electron microscopy, ELISA, and nanoparticle tracking analysis. The results show that hERG1 and Hsp47 were present in sEV membranes, extravesicularly exposing the sequences 430AFLLKETEEGPPATE445 for hERG1 and 169ALQSINEWAAQTT- DGKLPEVTKDVERTD196 for Hsp47. In addition, upon exposure to hypoxia, rat primary cardiomyocytes released sEVs into the media, and human cardiomyocytes in culture also released sEVs containing hERG1 (EV-hERG1) and/or Hsp47 (EV-Hsp47). Moreover, the levels of sEVs increased in the blood when cardiac ischemia was induced during the stress test, as well as the concentrations of EV-hERG1 and EV-Hsp47. Additionally, the plasma levels of EV-hERG1 and EV-Hsp47 decreased in patients with decompensated heart failure (DHF). Our data provide the first evidence that hERG1 and Hsp47 are present in the membranes of sEVs derived from the human cardiomyocyte cell line, and also in those isolated from human peripheral blood. Total sEVs, EV-hERG1, and EV-Hsp47 may be explored as biomarkers for heart diseases such as heart failure and cardiac ischemia.

Kcnh2 deletion is associated with rat embryonic development defects via destruction of KCNH2‑integrin β1 complex

The Kv11.1 potassium channel encoded by the Kcnh2 gene is crucial in conducting the rapid delayed rectifier K+ current in cardiomyocytes. Homozygous mutation in Kcnh2 is embryonically lethal in humans and mice. However, the molecular signaling pathway of intrauterine fetal loss is unclear. The present study generated a Kcnh2 knockout rat based on edited rat embryonic stem cells (rESCs). Kcnh2 knockout was embryonic lethal on day 11.5 of development due to a heart configuration defect. Experiments with human embryonic heart single cells (6.5‑7 weeks post‑conception) suggested that potassium voltage‑gated channel subfamily H member 2 (KCNH2) plays a crucial role in the development of compact cardiomyocytes. By contrast, apoptosis was found to be triggered in the homozygous embryos, which could be attributed to the failure of KCNH2 to form a complex with integrin β1 that was essential for preventing the process of apoptosis via inhibition of forkhead box O3A. Destruction of the KCNH2/integrin β1 complex reduced the phosphorylation level of AKT and deactivated the glycogen synthase kinase 3 β (GSK‑3β)/β‑catenin pathway, which caused early developmental abnormalities in rats. The present work reveals a basic mechanism by which KCNH2 maintains intact embryonic heart development.

KCNH2 encodes a nuclear-targeted polypeptide that mediates hERG1 channel gating and expression

KCNH2 encodes hERG1, the voltage-gated potassium channel that conducts the rapid delayed rectifier potassium current (I_Kr) in human cardiac tissue. hERG1 is one of the first channels expressed during early cardiac development, and its dysfunction is associated with intrauterine fetal death, sudden infant death syndrome, cardiac arrhythmia, and sudden cardiac death. Here, we identified a hERG1 polypeptide (hERG1_NP) that is targeted to the nuclei of immature cardiac cells, including human stem cell-derived cardiomyocytes (hiPSC-CMs) and neonatal rat cardiomyocytes. The nuclear hERG1_NP immunofluorescent signal is diminished in matured hiPSC-CMs and absent from adult rat cardiomyocytes. Antibodies targeting distinct hERG1 channel epitopes demonstrated that the hERG1_NP signal maps to the hERG1 distal C-terminal domain. KCNH2 deletion using CRISPR simultaneously abolished I_Kr and the hERG1_NP signal in hiPSC-CMs. We then identified a putative nuclear localization sequence (NLS) within the distal hERG1 C-terminus, 883-RQRKRKLSFR-892. Interestingly, the distal C-terminal domain was targeted almost exclusively to the nuclei when overexpressed HEK293 cells. Conversely, deleting the NLS from the distal peptide abolished nuclear targeting. Similarly, blocking α or β1 karyopherin activity diminished nuclear targeting. Finally, overexpressing the putative hERG1_NP peptide in the nuclei of HEK cells significantly reduced hERG1a current density, compared to cells expressing the NLS-deficient hERG1_NP or GFP. These data identify a developmentally regulated polypeptide encoded by KCNH2, hERG1_NP, whose presence in the nucleus indirectly modulates hERG1 current magnitude and kinetics.

Dynamics and physiological meaning of complexes between ion channels and integrin receptors: the case of Kv11.1

The cellular functions are regulated by a complex interplay of diffuse and local signals. Experimental work in cell physiology has led to recognize that understanding a cell's dynamics requires a deep comprehension of local fluctuations of cytosolic regulators. Macromolecular complexes are major determinants of local signaling. Multi-enzyme assemblies limit the diffusion restriction to reaction kinetics by direct exchange of metabolites. Likewise, close coupling of ion channels and transporters modulate the ion concentration around a channel mouth or transporter binding site. Extreme signal locality is brought about by conformational coupling between membrane proteins, as is typical of mechanotransduction. A paradigmatic case is integrin-mediated cell adhesion. Sensing the extracellular microenvironment and providing an appropriate response is essential in growth and development and has innumerable pathological implications. The process involves bidirectional signal transduction by complex supra-molecular structures that link integrin receptors to ion channels and transporters, growth factor receptors, cytoskeletal elements and other regulatory elements. The dynamics of such complexes is only beginning to be understood. A thoroughly studied example is the association between integrin receptors and the voltage-gated K+ channels Kv11.1. These channels are widely expressed in early embryos, where their physiological roles are poorly understood and apparently different from the shaping of action potential firing in the adult. Hints about these roles come from studies in cancer cells, where Kv11.1 is often overexpressed and appears to re-assume functions, such as controlling cell proliferation/differentiation, apoptosis and migration. Kv11.1 is implicated in these processes through its linking to integrin subunits.

Ion Channels and Transporters in Muscle Cell Differentiation

Investigations on ion channels in muscle tissues have mainly focused on physiological muscle function and related disorders, but emerging evidence supports a critical role of ion channels and transporters in developmental processes, such as controlling the myogenic commitment of stem cells. In this review, we provide an overview of ion channels and transporters that influence skeletal muscle myoblast differentiation, cardiac differentiation from pluripotent stem cells, as well as vascular smooth muscle cell differentiation. We highlight examples of model organisms or patients with mutations in ion channels. Furthermore, a potential underlying molecular mechanism involving hyperpolarization of the resting membrane potential and a series of calcium signaling is discussed.

Sodium Ions Do Not Stabilize the Selectivity Filter of a Potassium Channel

Ion conduction is an essential function for electrical activity in all organisms. The non-selective ion channel NaK was previously shown to adopt two stable conformations of the selectivity filter. Here, we present solid-state NMR measurements of NaK demonstrating a population shift between these conformations induced by changing the ions in the sample while the overall structure of NaK is not affected. We show that two K⁺-selective mutants (NaK2K and NaK2K-Y66F) suffer a complete loss of selectivity filter stability under Na⁺ conditions, but do not collapse into a defined structure. Widespread chemical shift perturbations are seen between the Na⁺ and K⁺ states of the K⁺-selective mutants in the region of the pore helix indicating structural changes. We conclude that the stronger link between the selectivity filter and the pore helix in the K⁺-selective mutants, compared to the non-selective wild-type NaK channel, reduces the ion-dependent conformational flexibility of the selectivity filter.

A Meta-Analysis of Bioelectric Data in Cancer, Embryogenesis, and Regeneration

Developmental bioelectricity is the study of the endogenous role of bioelectrical signaling in all cell types. Resting potentials and other aspects of ionic cell physiology are known to be important regulatory parameters in embryogenesis, regeneration, and cancer. However, relevant quantitative measurement and genetic phenotyping data are distributed throughout wide-ranging literature, hampering experimental design and hypothesis generation. Here, we analyze published studies on bioelectrics and transcriptomic and genomic/phenotypic databases to provide a novel synthesis of what is known in three important aspects of bioelectrics research. First, we provide a comprehensive list of channelopathies-ion channel and pump gene mutations-in a range of important model systems with developmental patterning phenotypes, illustrating the breadth of channel types, tissues, and phyla (including man) in which bioelectric signaling is a critical endogenous aspect of embryogenesis. Second, we perform a novel bioinformatic analysis of transcriptomic data during regeneration in diverse taxa that reveals an electrogenic protein to be the one common factor specifically expressed in regeneration blastemas across Kingdoms. Finally, we analyze data on distinct V_mem signatures in normal and cancer cells, revealing a specific bioelectrical signature corresponding to some types of malignancies. These analyses shed light on fundamental questions in developmental bioelectricity and suggest new avenues for research in this exciting field.

ERG1 plays an essential role in rat cardiomyocyte fate decision by mediating AKT signaling

ERG1, a potassium ion channel, is essential for cardiac action potential repolarization phase. However, the role of ERG1 for normal development of the heart is poorly understood. Using the rat embryonic stem cells (rESCs) model, we show that ERG1 is crucial in cardiomyocyte lineage commitment via interactions with Integrin β1. In the mesoderm phase of rESCs, the interaction of ERG1 with Integrin β1 can activate the AKT pathway by recruiting and phosphorylating PI3K p85 and focal adhesion kinase (FAK) to further phosphorylate AKT. Activation of AKT pathway promotes cardiomyocyte differentiation through two different mechanisms, (a) through phosphorylation of GSK3β to upregulate the expression levels of β-catenin and Gata4; (b) through promotion of nuclear translocation of nuclear factor-κB by phosphorylating IKKβ to inhibit cell apoptosis, which occurs due to increased Bcl2 expression. Our study provides solid evidence for a novel role of ERG1 on differentiation of rESCs into cardiomyocytes.

Kcnh2 mediates FAK/AKT-FOXO3A pathway to attenuate sepsis-induced cardiac dysfunction

Objectives

Myocardial dysfunction is a significant manifestation in sepsis, which results in high mortality. Even Kcnh2 has been hinted to associate with the pathological process, its involved signalling is still elusive.

Materials and methods

The caecal ligation puncture (CLP) surgery or lipopolysaccharide (LPS) injection was performed to induce septic cardiac dysfunction. Western blotting was used to determine KCNH2 expression. Cardiac function was examined by echocardiography 6 hours after CLP and LPS injection in Kcnh2 knockout (Kcnh2^+/‐) and NS1643 injection rats (n ≥ 6/group). Survival was monitored following CLP‐induced sepsis (n ≥ 8/group).

Results

Sepsis could downregulate KCNH2 level in the rat heart, as well as in LPS‐stimulated cardiomyocytes but not cardiac fibroblast. Defect of Kcnh2 (Kcnh2^+/‐) significantly aggravated septic cardiac dysfunction, exacerbated tissue damage and increased apoptosis under LPS challenge. Fractional shortening and ejection fraction values were significantly decreased in Kcnh2^+/‐ group than Kcnh2^+/+ group. Survival outcome in Kcnh2^+/‐ septic rats was markedly deteriorated, compared with Kcnh2^+/+ rats. Activated Kcnh2 with NS1643, however, resulted in opposite effects. Lack of Kcnh2 caused inhibition of FAK/AKT signalling, reflecting in an upregulation for FOXO3A and its downstream targets, which eventually induced cardiomyocyte apoptosis and heart tissue damage. Either activation of AKT by activator or knockdown of FOXO3A with si‐RNA remarkably attenuated the pathological manifestations that Kcnh2 defect mediated.

Conclusion

Kcnh2 plays a protection role in sepsis‐induced cardiac dysfunction (SCID) via regulating FAK/AKT‐FOXO3A to block LPS‐induced myocardium apoptosis, indicating a potential effect of the potassium channels in pathophysiology of SCID.

Identification and Characterization of a Transcribed Distal Enhancer Involved in Cardiac Kcnh2 Regulation

The human ether-a-go-go-related gene KCNH2 encodes the voltage-gated potassium channel underlying I_Kr, a current critical for the repolarization phase of the cardiac action potential. Mutations in KCNH2 that cause a reduction of the repolarizing current can result in cardiac arrhythmias associated with long-QT syndrome. Here, we investigate the regulation of KCNH2 and identify multiple active enhancers. A transcribed enhancer ∼85 kbp downstream of Kcnh2 physically contacts the promoters of two Kcnh2 isoforms in a cardiac-specific manner in vivo. Knockdown of its ncRNA transcript results in reduced expression of Kcnh2b and two neighboring mRNAs, Nos3 and Abcb8, in vitro. Genomic deletion of the enhancer, including the ncRNA transcription start site, from the mouse genome causes a modest downregulation of both Kcnh2a and Kcnh2b in the ventricles. These findings establish that the regulation of Kcnh2a and Kcnh2b is governed by a complex regulatory landscape that involves multiple partially redundantly acting enhancers.

Selectivity filter modalities and rapid inactivation of the hERG1 channel

The human ether-á-go-go-related gene (hERG1) channel conducts small outward K⁺ currents that are critical for cardiomyocyte membrane repolarization. The gain-of-function mutation N629D at the outer mouth of the selectivity filter (SF) disrupts inactivation and K⁺-selective transport in hERG1, leading to arrhythmogenic phenotypes associated with long-QT syndrome. Here, we combined computational electrophysiology with Markov state model analysis to investigate how SF-level gating modalities control selective cation transport in wild-type (WT) and mutant (N629D) hERG1 variants. Starting from the recently reported cryogenic electron microscopy (cryo-EM) open-state channel structure, multiple microseconds-long molecular-dynamics (MD) trajectories were generated using different cation configurations at the filter, voltages, electrolyte concentrations, and force-field parameters. Most of the K⁺ permeation events observed in hERG1-WT simulations occurred at microsecond timescales, influenced by the spontaneous dehydration/rehydration dynamics at the filter. The SF region displayed conductive, constricted, occluded, and dilated states, in qualitative agreement with the well-documented flickering conductance of hERG1. In line with mutagenesis studies, these gating modalities resulted from dynamic interaction networks involving residues from the SF, outer-mouth vestibule, P-helices, and S5-P segments. We found that N629D mutation significantly stabilizes the SF in a state that is permeable to both K⁺ and Na⁺, which is reminiscent of the SF in the nonselective bacterial NaK channel. Increasing the external K⁺ concentration induced "WT-like" SF dynamics in N629D, in qualitative agreement with the recovery of flickering currents in experiments. Overall, our findings provide an understanding of the molecular mechanisms controlling selective transport in K⁺ channels with a nonconventional SF sequence.

Planarian regeneration as a model of anatomical homeostasis: Recent progress in biophysical and computational approaches

Planarian behavior, physiology, and pattern control offer profound lessons for regenerative medicine, evolutionary biology, morphogenetic engineering, robotics, and unconventional computation. Despite recent advances in the molecular genetics of stem cell differentiation, this model organism's remarkable anatomical homeostasis provokes us with truly fundamental puzzles about the origin of large-scale shape and its relationship to the genome. In this review article, we first highlight several deep mysteries about planarian regeneration in the context of the current paradigm in this field. We then review recent progress in understanding of the physiological control of an endogenous, bioelectric pattern memory that guides regeneration, and how modulating this memory can permanently alter the flatworm's target morphology. Finally, we focus on computational approaches that complement reductive pathway analysis with synthetic, systems-level understanding of morphological decision-making. We analyze existing models of planarian pattern control and highlight recent successes and remaining knowledge gaps in this interdisciplinary frontier field.

Selective Serotonin Reuptake Inhibitor Use During Pregnancy and Major Malformations: The Importance of Serotonin for Embryonic Development and the Effect of Serotonin Inhibition on the Occurrence of Malformations

Bioelectric signaling is transduced by neurotransmitter pathways in many cell types. One of the key mediators of bioelectric control mechanisms is serotonin, and its transporter SERT, which is targeted by a broad class of blocker drugs (selective serotonin reuptake inhibitors [SSRIs]). Studies showing an increased risk of multiple malformations associated with gestational use of SSRI have been accumulating but debate remains on whether SSRI as a class has the potential to generate these malformations. This review highlights the importance of serotonin for embryonic development; the effect of serotonin inhibition during early pregnancy on the occurrence of multiple diverse malformations that have been shown to occur in human pregnancies; that the risks outweigh the benefits of SSRI use during gestation in populations of mild to moderately depressed pregnant women, which encompass the majority of pregnant depressed women; and that the malformations seen in human pregnancies constitute a pattern of malformations consistent with the known mechanisms of action of SSRIs. We present at least three mechanisms by which SSRI can affect development. These studies highlight the relevance of basic bioelectric and neurotransmitter mechanism for biomedicine.

Human ether‑à‑go‑go‑related gene mutation L539fs/47‑hERG leads to cell apoptosis through the endoplasmic reticulum stress pathway

Congenital long QT syndrome (LQTS) is a cardiac channelopathy that often results in fatal arrhythmias. LQTS mutations not only lead to abnormal myocardial electrical activities but are associated with heart contraction abnormalities, cardiomyopathy and congenital heart defects. In vivo and in vitro studies have found that LQTS mutations are associated with cardiomyocyte apoptosis, cardiac developmental disorders and even embryonic mortality. Cardiac delayed rectifier potassium channel dysfunction due to the human ether‑à‑go‑go‑related gene (hERG) mutation causes congenital LQTS type 2. The majority of LQTS 2 mutations are characterized by mutant protein accumulation in the endoplasmic reticulum (ER). Unfolded or misfolded protein retention in the ER causes an unfolded protein reaction, which is characteristic of ER stress (ERS). Therefore, the present study hypothesized that LQTS mutations can cause cardiac structural abnormalities via ERS‑mediated cardiomyocyte apoptosis. To test this hypothesis, 293 cells were transiently transfected with an L539fs/47‑hERG plasmid to generate an LQTS 2 model. L539fs/47‑hERG is an LQTS 2 mutation, which consists of a 19‑bp deletion at 1619‑1637 and a point mutation at 1692. Using confocal laser scanning microscopy analysis, it was verified that the L539fs/47‑hERG protein was retained in the ER. Hoechst 33342 apoptosis staining indicated that apoptosis was increased in the L539fs/47‑hERG‑transfected cells, and this be reversed by treatment with 4‑phenyl butyric acid. Western blot analysis revealed increased expression levels of the ERS chaperone glucose regulated protein 78 and pro‑apoptotic ERS‑induced factors, including protein kinase R‑like endoplasmic reticulum kinase, eukaryotic translation‑initiation factor‑2α and C/EBP homologous protein, in the L539fs/47‑hERG‑transfected cells. The B‑cell lymphoma (Bcl‑2)‑associated X protein/Bcl‑2 ratio and caspase‑12 were also increased in the mutated cells. These results demonstrate that L539fs/47‑hERG induces cell apoptosis and the potential molecular mechanism involves the activation of ERS and ERS‑mediated cell apoptosis.

Ondansetron and teratogenicity in rats: Evidence for a mechanism mediated via embryonic hERG blockade

The potent hERG channel blocking drug ondansetron is used off-label for treatment of nausea and vomiting in early pregnancy. Some human epidemiological studies have associated ondansetron with fetal cardiovascular defects and orofacial clefts. This study investigated the effects of ondanestron on embryonic heart rhythm of gestational day (GD) 13 rat embryos in vitro and then integrated the results with published animal teratology, and animal and human pharmacokinetic studies to perform a risk evaluation. Ondansetron caused concentration dependent bradycardia and arrhythmia. Cardiovascular malformations in rats occurred at exposures slightly higher than those in early human pregnancy. Together the results suggest that ondansetron can have teratogenic potential in rats and humans mediated via hERG block and severe heart rhythm disturbances in the embryo. The risk may be increased in human pregnancy if additional risk factors are present such as hypokalemia.

Modulation of Ether-à-Go-Go Related Gene (ERG) Current Governs Intrinsic Persistent Activity in Rodent Neocortical Pyramidal Cells

While cholinergic receptor activation has long been known to dramatically enhance the excitability of cortical neurons, the cellular mechanisms responsible for this effect are not well understood. We used intracellular recordings in rat (both sexes) neocortical brain slices to assess the ionic mechanisms supporting persistent firing modes triggered by depolarizing stimuli following cholinergic receptor activation. We found multiple lines of evidence suggesting that a component of the underlying hyperexcitability associated with persistent firing reflects a reduction in the standing (leak) K⁺ current mediated by Ether-a-go-go-Related Gene (ERG) channels. Three chemically diverse ERG channel blockers (terfenadine, ErgToxin-1, and E-4031) abolished persistent firing and the underlying increase in input resistance in deep pyramidal cells in temporal and prefrontal association neocortex. Calcium accumulation during triggering stimuli appears to attenuate ERG currents, leading to membrane potential depolarization and increased input resistance, two critical elements generating persistent firing. Our results also suggest that ERG current normally governs cortical neuron responses to depolarizing stimuli by opposing prolonged discharges and by enhancing the poststimulus repolarization. The broad expression of ERG channels and the ability of ERG blocks to abolish persistent firing evoked by both synaptic and intracellular step stimuli suggest that modulation of ERG channels may underlie many forms of persistent activity observed in vivoSIGNIFICANCE STATEMENT Persistent activity, where spiking continues beyond the triggering stimulus, is a common phenomenon observed in many types of neurons. Identifying the mechanism underlying this elementary process of memory is a step forward in understanding higher cognitive function including short-term memory. Our results suggest that a reduction in the currents normally mediated by Ether-a-go-go-Related Gene (ERG) K⁺ channels contributes to persistent firing in neocortical pyramidal cells. ERG currents have been previously studied primarily in the heart; relatively little is known about ERG function in the brain, although mutations in ERG channels have recently been linked to schizophrenia. The present study is among the first to describe its role in neocortex in relation to biophysical correlates of memory function.

Cardiotoxic effect of levofloxacin and ciprofloxacin in rats with/without acute myocardial infarction: Impact on cardiac rhythm and cardiac expression of Kv4.3, Kv1.2 and Nav1.5 channels

Prolongation of QT interval is possible with fluoroquinolones, yet the underlying contributing factors have not been elucidated. Two widely used fluoroquinolone drugs were at the focus of this study in rats with/without acute myocardial dysfunction (AMI) induced by isoproterenol. The effects of levofloxacin and ciprofloxacin on the cardiac mRNA expression of rat Kv4.3, Kv1.2 and Nav1.5 mRNAs were determined. Administration of the two antibiotics produced dose-dependent changes in ECG parameters that were more prominent in rats with AMI than healthy rats; this was accompanied by elevations in serum lactate dehydrogenase and creatine kinase-MB. Histopathological examination indicated some loss of striations, edema and fibrotic changes in rats with AMI; however the two antibiotics did not further exacerbate the cardiac histopathology. mRNA expression of the ion channels was altered in rats with AMI and healthy rats. In conclusion, long-term administration of levofloxacin and ciprofloxacin produced deleterious effects on the ECG pattern of rats with/without AMI. The effect was generally baseline-dependent and therefore, rats with AMI showed greater ECG disturbances and increases in cardiac enzymes. Taken together, these data make it advisable to monitor patients with a history of acute AMI requiring treatment with these antibiotics until data from human studies are available.

Age-dependent electrical and morphological remodeling of the Drosophila heart caused by hERG/seizure mutations

Understanding the cellular-molecular substrates of heart disease is key to the development of cardiac specific therapies and to the prevention of off-target effects by non-cardiac targeted drugs. One of the primary targets for therapeutic intervention has been the human ether a go-go (hERG) K⁺ channel that, together with the KCNQ channel, controls the rate and efficiency of repolarization in human myocardial cells. Neither of these channels plays a major role in adult mouse heart function; however, we show here that the hERG homolog seizure (sei), along with KCNQ, both contribute significantly to adult heart function as they do in humans. In Drosophila, mutations in or cardiac knockdown of sei channels cause arrhythmias that become progressively more severe with age. Intracellular recordings of semi-intact heart preparations revealed that these perturbations also cause electrical remodeling that is reminiscent of the early afterdepolarizations seen in human myocardial cells defective in these channels. In contrast to KCNQ, however, mutations in sei also cause extensive structural remodeling of the myofibrillar organization, which suggests that hERG channel function has a novel link to sarcomeric and myofibrillar integrity. We conclude that deficiency of ion channels with similar electrical functions in cardiomyocytes can lead to different types or extents of electrical and/or structural remodeling impacting cardiac output.

We have used the fruit fly cardiac model to show that seizure, the fly homolog of the human ether a go-go K⁺ channel hERG, is functional in the fly heart. This channel plays a major role in cardiac repolarization in humans but not in adult rodent hearts. Loss of channel function in the fly causes bradycardia, electrical arrhythmia and altered myofibrillar structure. Gene expression analysis indicates that Wnt signaling is affected and we show a genetic interaction between sei and pygopus, a Wnt pathway component, on heart function.

hERG1 potassium channel in cancer cells: a tool to reprogram immortality

It has been well established that changes in ion fluxes across cellular membranes as a function of time is fundamental in maintaining cellular homeostasis of every living cell. Consequently, dysregulation of ion channels activity is a critical event in pathological conditions of several tissues, including cancer. Nevertheless, the role of ion channels in cancer biology is still not well understood and very little is known about the possible therapeutic opportunities offered by the use of the vast collection of drugs that target ion channels. In this review, we focus on the recent advances in understanding the role of the voltage-gated hERG1 potassium channel in cancer and on the effects of pharmacologic manipulation of the hERG1 in cancer cells aiming to provide insights into the biochemical signaling and cellular processes that are altered by using these drugs.

The electrophysiological development of cardiomyocytes

The generation of human cardiomyocytes (CMs) from human pluripotent stem cells (hPSCs) has become an important resource for modeling human cardiac disease and for drug screening, and also holds significant potential for cardiac regeneration. Many challenges remain to be overcome however, before innovation in this field can translate into a change in the morbidity and mortality associated with heart disease. Of particular importance for the future application of this technology is an improved understanding of the electrophysiologic characteristics of CMs, so that better protocols can be developed and optimized for generating hPSC-CMs. Many different cell culture protocols are currently utilized to generate CMs from hPSCs and all appear to yield relatively “developmentally” immature CMs with highly heterogeneous electrical properties. These hPSC-CMs are characterized by spontaneous beating at highly variable rates with a broad range of depolarization-repolarization patterns, suggestive of mixed populations containing atrial, ventricular and nodal cells. Many recent studies have attempted to introduce approaches to promote maturation and to create cells with specific functional properties. In this review, we summarize the studies in which the electrical properties of CMs derived from stem cells have been examined. In order to place this information in a useful context, we also review the electrical properties of CMs as they transition from the developing embryo to the adult human heart. The signal pathways involved in the regulation of ion channel expression during development are also briefly considered.

Molecular bioelectricity: how endogenous voltage potentials control cell behavior and instruct pattern regulation in vivo

In addition to biochemical gradients and transcriptional networks, cell behavior is regulated by endogenous bioelectrical cues originating in the activity of ion channels and pumps, operating in a wide variety of cell types. Instructive signals mediated by changes in resting potential control proliferation, differentiation, cell shape, and apoptosis of stem, progenitor, and somatic cells. Of importance, however, cells are regulated not only by their own V_mem but also by the V_mem of their neighbors, forming networks via electrical synapses known as gap junctions. Spatiotemporal changes in V_mem distribution among nonneural somatic tissues regulate pattern formation and serve as signals that trigger limb regeneration, induce eye formation, set polarity of whole-body anatomical axes, and orchestrate craniofacial patterning. New tools for tracking and functionally altering V_mem gradients in vivo have identified novel roles for bioelectrical signaling and revealed the molecular pathways by which V_mem changes are transduced into cascades of downstream gene expression. Because channels and gap junctions are gated posttranslationally, bioelectrical networks have their own characteristic dynamics that do not reduce to molecular profiling of channel expression (although they couple functionally to transcriptional networks). The recent data provide an exciting opportunity to crack the bioelectric code, and learn to program cellular activity at the level of organs, not only cell types. The understanding of how patterning information is encoded in bioelectrical networks, which may require concepts from computational neuroscience, will have transformative implications for embryogenesis, regeneration, cancer, and synthetic bioengineering.

Role of Mutation and Pharmacologic Block of Human KCNH2 in Vasculogenesis and Fetal Mortality

Background—

N629D KCNH2 is a human missense long-QT2 mutation. Previously, we reported that the N629D/N629D mutation embryos disrupted cardiac looping, right ventricle development, and ablated I Kr activity at E9.5. The present study evaluates the role of KCNH2 in vasculogenesis.

Methods and Results—

N629D/N629D yolk sac vessels and aorta consist of sinusoids without normal arborization. Isolated E9.5 +/+ first branchial arches showed normal outgrowth of mouse ERG–positive/α-smooth muscle actin coimmunolocalized cells; however, outgrowth was grossly reduced in N629D/N629D. N629D/N629D aortas showed fewer α-smooth muscle actin positive cells that were not coimmunolocalized with mouse ERG cells. Transforming growth factor-β treatment of isolated N629D/N629D embryoid bodies partially rescued this phenotype. Cultured N629D/N629D embryos recapitulate the same cardiovascular phenotypes as seen in vivo. Transforming growth factor-β treatment significantly rescued these embryonic phenotypes. Both in vivo and in vitro, dofetilide treatment, over a narrow window of time, entirely recapitulated the N629D/N629D fetal phenotypes. Exogenous transforming growth factor-β treatment also rescued the dofetilide-induced phenotype toward normal.

Conclusions—

Loss of function of KCNH2 mutations results in defects in cardiogenesis and vasculogenesis. Because many medications inadvertently block the KCNH2 potassium current, these novel findings seem to have clinical relevance.

Electrophysiologic substrate in congenital Long QT syndrome: noninvasive mapping with electrocardiographic imaging (ECGI)

BACKGROUND

Congenital Long QT syndrome (LQTS) is an arrhythmogenic disorder that causes syncope and sudden death. Although its genetic basis has become well-understood, the mechanisms whereby mutations translate to arrhythmia susceptibility in the in situ human heart have not been fully defined. We used noninvasive ECG imaging to map the cardiac electrophysiological substrate and examine whether LQTS patients display regional heterogeneities in repolarization, a substrate that promotes arrhythmogenesis.

METHODS AND RESULTS

Twenty-five subjects (9 LQT1, 9 LQT2, 5 LQT3, and 2 LQT5) with genotype and phenotype positive LQTS underwent ECG imaging. Seven normal subjects provided control. Epicardial maps of activation, recovery times, activation-recovery intervals, and repolarization dispersion were constructed. Activation was normal in all patients. However, recovery times and activation-recovery intervals were prolonged relative to control, indicating delayed repolarization and abnormally long action potential duration (312±30 ms versus 235±21 ms in control). Activation-recovery interval prolongation was spatially heterogeneous, with repolarization gradients much steeper than control (119±19 ms/cm versus 2.0±2.0 ms/cm). There was variability in steepness and distribution of repolarization gradients between and within LQTS types. Repolarization gradients were steeper in symptomatic patients (130±27 ms/cm in 12 symptomatic patients versus 98±19 ms/cm in 13 asymptomatic patients; P<0.05).

CONCLUSIONS

LQTS patients display regions with steep repolarization dispersion caused by localized action potential duration prolongation. This defines a substrate for reentrant arrhythmias, not detectable by surface ECG. Steeper dispersion in symptomatic patients suggests a possible role for ECG imaging in risk stratification.

Elevated heart rate triggers action potential alternans and sudden death. translational study of a homozygous KCNH2 mutation

BACKGROUND

Long QT syndrome (LQTS) leads to arrhythmic events and increased risk for sudden cardiac death (SCD). Homozygous KCNH2 mutations underlying LQTS-2 have previously been termed "human HERG knockout" and typically express severe phenotypes. We studied genotype-phenotype correlations of an LQTS type 2 mutation identified in the homozygous index patient from a consanguineous Turkish family after his brother died suddenly during febrile illness.

METHODS AND RESULTS

Clinical work-up, DNA sequencing, mutagenesis, cell culture, patch-clamp, in silico mathematical modelling, protein biochemistry, confocal microscopy were performed. Genetic analysis revealed a homozygous C-terminal KCNH2 mutation (p.R835Q) in the index patient (QTc ∼506 ms with notched T waves). Parents were I° cousins - both heterozygous for the mutation and clinically unremarkable (QTc ∼447 ms, father and ∼396 ms, mother). Heterologous expression of KCNH2-R835Q showed mildly reduced current amplitudes. Biophysical properties of ionic currents were also only nominally changed with slight acceleration of deactivation and more negative V50 in R835Q-currents. Protein biochemistry and confocal microscopy revealed similar expression patterns and trafficking of WT and R835Q, even at elevated temperature. In silico analysis demonstrated mildly prolonged ventricular action potential duration (APD) compared to WT at a cycle length of 1000 ms. At a cycle length of 350 ms M-cell APD remained stable in WT, but displayed APD alternans in R835Q.

CONCLUSION

Kv11.1 channels affected by the C-terminal R835Q mutation display mildly modified biophysical properties, but leads to M-cell APD alternans with elevated heart rate and could precipitate SCD under specific clinical circumstances associated with high heart rates.

The effect on rat embryonic heart rate of Na+, K+, and Ca2+ channel blockers, and the human teratogen phenytoin, changes with gestational age

In this study, we compared the effects of four ion channel blockers on rat embryonic heart function during the organogenic period from gestational day (GD) 10 to 15, to determine the changes in dependence on ion channels during rat cardiac development. Rat embryos in culture were exposed to either the human ether-á-go-go-related gene potassium channel blocker, dofetilide (400 nM); the sodium channel blocker, lidocaine (250 μM); the L-type calcium channel blocker, nifedipine (1.8 μM); or the multichannel blocker, phenytoin (200 μM). Lidocaine slowed the heart rate (HR) with the effect becoming more severe with increasing GD. Dofetilide slowed the embryonic HR and caused arrhythmias with the most severe effect on GD 11 to 13. Nifedipine primarily caused a negative inotropic effect except on GD 10 when it stopped the heart in most embryos. Phenytoin stopped the heart of most GD 10 to 12 embryos while on GD 13 to 15 phenytoin slowed the heart. The results demonstrate that as the rat heart develops during the organogenic period its functional dependence on ion channels changes markedly. These changes are important for understanding drug effects on the embryo during pregnancy and the methodology used provides a simple procedure for assessing drug effects on the developing heart.

Reprogramming cells and tissue patterning via bioelectrical pathways: molecular mechanisms and biomedical opportunities

Transformative impact in regenerative medicine requires more than the reprogramming of individual cells: advances in repair strategies for birth defects or injuries, tumor normalization, and the construction of bioengineered organs and tissues all require the ability to control large-scale anatomical shape. Much recent work has focused on the transcriptional and biochemical regulation of cell behavior and morphogenesis. However, exciting new data reveal that bioelectrical properties of cells and their microenvironment exert a profound influence on cell differentiation, proliferation, and migration. Ion channels and pumps expressed in all cells, not just excitable nerve and muscle, establish resting potentials that vary across tissues and change with significant developmental events. Most importantly, the spatiotemporal gradients of these endogenous transmembrane voltage potentials (Vmem ) serve as instructive patterning cues for large-scale anatomy, providing organ identity, positional information, and prepattern template cues for morphogenesis. New genetic and pharmacological techniques for molecular modulation of bioelectric gradients in vivo have revealed the ability to initiate complex organogenesis, change tissue identity, and trigger regeneration of whole vertebrate appendages. A large segment of the spatial information processing that orchestrates individual cells' programs toward the anatomical needs of the host organism is electrical; this blurs the line between memory and decision-making in neural networks and morphogenesis in nonneural tissues. Advances in cracking this bioelectric code will enable the rational reprogramming of shape in whole tissues and organs, revolutionizing regenerative medicine, developmental biology, and synthetic bioengineering.

Endogenous voltage gradients as mediators of cell-cell communication: strategies for investigating bioelectrical signals during pattern formation

Alongside the well-known chemical modes of cell-cell communication, we find an important and powerful system of bioelectrical signaling: changes in the resting voltage potential (Vmem) of the plasma membrane driven by ion channels, pumps and gap junctions. Slow Vmem changes in all cells serve as a highly conserved, information-bearing pathway that regulates cell proliferation, migration and differentiation. In embryonic and regenerative pattern formation and in the disorganization of neoplasia, bioelectrical cues serve as mediators of large-scale anatomical polarity, organ identity and positional information. Recent developments have resulted in tools that enable a high-resolution analysis of these biophysical signals and their linkage with upstream and downstream canonical genetic pathways. Here, we provide an overview for the study of bioelectric signaling, focusing on state-of-the-art approaches that use molecular physiology and developmental genetics to probe the roles of bioelectric events functionally. We highlight the logic, strategies and well-developed technologies that any group of researchers can employ to identify and dissect ionic signaling components in their own work and thus to help crack the bioelectric code. The dissection of bioelectric events as instructive signals enabling the orchestration of cell behaviors into large-scale coherent patterning programs will enrich on-going work in diverse areas of biology, as biophysical factors become incorporated into our systems-level understanding of cell interactions.

hERG channel function: beyond long QT

To date, research on the human ether-a-go-go related gene (hERG) has focused on this potassium channel's role in cardiac repolarization and Long QT Syndrome (LQTS). However, growing evidence implicates hERG in a diversity of physiologic and pathological processes. Here we discuss these other functions of hERG, particularly their impact on diseases beyond cardiac arrhythmia.

Endogenous Voltage Potentials and the Microenvironment: Bioelectric Signals that Reveal, Induce and Normalize Cancer

Cancer may be a disease of geometry: a misregulation of the field of information that orchestrates individual cells' activities towards normal anatomy. Recent work identified molecular mechanisms underlying a novel system of developmental control: bioelectric gradients. Endogenous spatio-temporal differences in resting potential of non-neural cells provide instructive cues for cell regulation and complex patterning during embryogenesis and regeneration. It is now appreciated that these cues are an important layer of the dysregulation of cell: cell interactions that leads to cancer. Abnormal depolarization of resting potential (Vmem) is a convenient marker for neoplasia and activates a metastatic phenotype in genetically-normal cells in vivo. Moreover, oncogene expression depolarizes cells that form tumor-like structures, but is unable to form tumors if this depolarization is artificially prevented by misexpression of hyperpolarizing ion channels. Vmem triggers metastatic behaviors at considerable distance, mediated by transcriptional and epigenetic effects of electrically-modulated flows of serotonin and butyrate. While in vivo data on voltages in carcinogenesis comes mainly from the amphibian model, unbiased genetic screens and network profiling in rodents and human tissues reveal several ion channel proteins as bona fide oncogene and promising targets for cancer drug development. However, we propose that a focus on specific channel genes is just the tip of the iceberg. Bioelectric state is determined by post-translational gating of ion channels, not only from genetically-specified complements of ion translocators. A better model is a statistical dynamics view of spatial Vmem gradients. Cancer may not originate at the single cell level, since gap junctional coupling results in multi-cellular physiological networks with multiple stable attractors in bioelectrical state space. New medical applications await a detailed understanding of the mechanisms by which organ target morphology stored in real-time patterns of ion flows is perceived or mis-perceived by cells. Mastery of somatic voltage gradients will lead to cancer normalization or rebooting strategies, such as those that occur in regenerating and embryonic organs, resulting in transformative advances in basic biology and oncology.

hERG K+ Channels: Structure, Function, and Clinical Significance

The human ether-a-go-go related gene (hERG) encodes the pore-forming subunit of the rapid component of the delayed rectifier K+ channel, Kv11.1, which are expressed in the heart, various brain regions, smooth muscle cells, endocrine cells, and a wide range of tumor cell lines. However, it is the role that Kv11.1 channels play in the heart that has been best characterized, for two main reasons. First, it is the gene product involved in chromosome 7-associated long QT syndrome (LQTS), an inherited disorder associated with a markedly increased risk of ventricular arrhythmias and sudden cardiac death. Second, blockade of Kv11.1, by a wide range of prescription medications, causes drug-induced QT prolongation with an increase in risk of sudden cardiac arrest. In the first part of this review, the properties of Kv11.1 channels, including biogenesis, trafficking, gating, and pharmacology are discussed, while the second part focuses on the pathophysiology of Kv11.1 channels.

Molecular bioelectricity in developmental biology: new tools and recent discoveries: control of cell behavior and pattern formation by transmembrane potential gradients

Significant progress in the molecular investigation of endogenous bioelectric signals during pattern formation in growing tissues has been enabled by recently developed techniques. Ion flows and voltage gradients produced by ion channels and pumps are key regulators of cell proliferation, migration, and differentiation. Now, instructive roles for bioelectrical gradients in embryogenesis, regeneration, and neoplasm are being revealed through the use of fluorescent voltage reporters and functional experiments using well-characterized channel mutants. Transmembrane voltage gradients (V(mem) ) determine anatomical polarity and function as master regulators during appendage regeneration and embryonic left-right patterning. A state-of-the-art recent study reveals that they can also serve as prepatterns for gene expression domains during craniofacial patterning. Continued development of novel tools and better ways to think about physical controls of cell-cell interactions will lead to mastery of the morphogenetic information stored in physiological networks. This will enable fundamental advances in basic understanding of growth and form, as well as transformative biomedical applications in regenerative medicine.

Regulation of cell behavior and tissue patterning by bioelectrical signals: challenges and opportunities for biomedical engineering

Achieving control over cell behavior and pattern formation requires molecular-level understanding of regulatory mechanisms. Alongside transcriptional networks and biochemical gradients, there functions an important system of cellular communication and control: transmembrane voltage gradients (V(mem)). Bioelectrical signals encoded in spatiotemporal changes of V(mem) control cell proliferation, migration, and differentiation. Moreover, endogenous bioelectrical gradients serve as instructive cues mediating anatomical polarity and other organ-level aspects of morphogenesis. In the past decade, significant advances in molecular physiology have enabled the development of new genetic and biophysical tools for the investigation and functional manipulation of bioelectric cues. Recent data implicate V(mem) as a crucial epigenetic regulator of patterning events in embryogenesis, regeneration, and cancer. We review new conceptual and methodological developments in this fascinating field. Bioelectricity offers a novel way of quantitatively understanding regulation of growth and form in vivo, and it reveals tractable, powerful control points that will enable truly transformative applications in bioengineering, regenerative medicine, and synthetic biology.

Progesterone impairs human ether-a-go-go-related gene (HERG) trafficking by disruption of intracellular cholesterol homeostasis

The prolongation of QT intervals in both mothers and fetuses during the later period of pregnancy implies that higher levels of progesterone may regulate the function of the human ether-a-go-go-related gene (HERG) potassium channel, a key ion channel responsible for controlling the length of QT intervals. Here, we studied the effect of progesterone on the expression, trafficking, and function of HERG channels and the underlying mechanism. Treatment with progesterone for 24 h decreased the abundance of the fully glycosylated form of the HERG channel in rat neonatal cardiac myocytes and HERG-HEK293 cells, a cell line stably expressing HERG channels. Progesterone also concentration-dependently decreased HERG current density, but had no effect on voltage-gated L-type Ca(2+) and K(+) channels. Immunofluorescence microscopy and Western blot analysis show that progesterone preferentially decreased HERG channel protein abundance in the plasma membrane, induced protein accumulation in the dilated endoplasmic reticulum (ER), and increased the protein expression of C/EBP homologous protein, a hallmark of ER stress. Application of 2-hydroxypropyl-β-cyclodextrin (a sterol-binding agent) or overexpression of Rab9 rescued the progesterone-induced HERG trafficking defect and ER stress. Disruption of intracellular cholesterol homeostasis with simvastatin, imipramine, or exogenous application of cholesterol mimicked the effect of progesterone on HERG channel trafficking. Progesterone may impair HERG channel folding in the ER and/or block its trafficking to the Golgi complex by disrupting intracellular cholesterol homeostasis. Our findings may reveal a novel molecular mechanism to explain the QT prolongation and high risk of developing arrhythmias during late pregnancy.

Probing human cardiovascular congenital disease using transgenic mouse models

Congenital heart defects (CHDs) impact in utero embryonic viability, children, and surviving adults. Since the first transfer of genes into mice, transgenic mouse models have enabled researchers to experimentally study and genetically test the roles of genes in development, physiology, and disease progression. Transgenic mice have become a bona fide human CHD pathology model and their use has dramatically increased within the past two decades. Now that the entire mouse and human genomes are known, it is possible to knock out, mutate, misexpress, and/or replace every gene. Not only have transgenic mouse models changed our understanding of normal development, CHD processes, and the complex interactions of genes and pathways required during heart development, but they are also being used to identify new avenues for medical therapy.

Properties of WT and mutant hERG K(+) channels expressed in neonatal mouse cardiomyocytes

Mutations in human ether-a-go-go-related gene 1 (hERG) are linked to long QT syndrome type 2 (LQT2). hERG encodes the pore-forming alpha-subunits that coassemble to form rapidly activating delayed rectifier K(+) current in the heart. LQT2-linked missense mutations have been extensively studied in noncardiac heterologous expression systems, where biogenic (protein trafficking) and biophysical (gating and permeation) abnormalities have been postulated to underlie the loss-of-function phenotype associated with LQT2 channels. Little is known about the properties of LQT2-linked hERG channel proteins in native cardiomyocyte systems. In this study, we expressed wild-type (WT) hERG and three LQT2-linked mutations in neonatal mouse cardiomyocytes and studied their electrophysiological and biochemical properties. Compared with WT hERG channels, the LQT2 missense mutations G601S and N470D hERG exhibited altered protein trafficking and underwent pharmacological correction, and N470D hERG channels gated at more negative voltages. The DeltaY475 hERG deletion mutation trafficked similar to WT hERG channels, gated at more negative voltages, and had rapid deactivation kinetics, and these properties were confirmed in both neonatal mouse cardiomyocyte and human embryonic kidney (HEK)-293 cell expression systems. Differences between the cardiomyocytes and HEK-293 cell expression systems were that hERG current densities were reduced 10-fold and deactivation kinetics were accelerated 1.5- to 2-fold in neonatal mouse cardiomyocytes. An important finding of this work is that pharmacological correction of trafficking-deficient LQT2 mutations, as a potential innovative approach to therapy, is possible in native cardiac tissue.

HERG1 channelopathies

Human ether a go-go-related gene type 1 (hERG1) K+ channels conduct the rapid delayed rectifier K+ current and mediate action potential repolarization in the heart. Mutations in KCNH2 (the gene that encodes hERG1) causes LQT2, one of the most common forms of long QT syndrome, a disorder of cardiac repolarization that predisposes affected subjects to ventricular arrhythmia and increases the risk of sudden cardiac death. Hundreds of LQT2-associated mutations have been described, and most cause a loss of function by disrupting subunit folding, assembly, or trafficking of the channel to the cell surface. Loss-of-function mutations in hERG1 channels have also recently been implicated in epilepsy. A single gain-of-function mutation has been described that causes short QT syndrome and cardiac arrhythmia. In addition, up-regulation of hERG1 channel expression has been demonstrated in specific tumors and has been associated with skeletal muscle atrophy in mice.

Sex-dependent gene regulatory networks of the heart rhythm

Expression level, control, and intercoordination of 66 selected heart rhythm determinant (HRD) genes were compared in atria and ventricles of four male and four female adult mice. We found that genes encoding various adrenergic receptors, ankyrins, ion channels and transporters, connexins, cadherins, plakophilins, and other components of the intercalated discs form a complex network that is chamber dependent and differs between the two sexes. In addition, most HRD genes in atria had higher expression in males than in females, while in ventricles, expression levels were mostly higher in females than in males. Moreover, significant chamber differences were observed between the sexes, with higher expression in atria than ventricles for males and higher expression in ventricles than atria for females. We have ranked the selected genes according to their prominence (new concept) within the HRD gene web defined as extent of expression coordination with the other web genes and stability of expression. Interestingly, the prominence hierarchy was substantially different between the two sexes. Taken together, these findings indicate that the organizational principles of the heart rhythm transcriptome are sex dependent, with the newly introduced prominence analysis allowing identification of genes that are pivotal for the sexual dichotomy.