Natural products modulating the hERG channel: heartaches and hope.

Covering: 1996-December 2016The human Ether-à-go-go Related Gene (hERG) channel is a voltage-gated potassium channel playing an essential role in the normal electrical activity in the heart. It is involved in the repolarization and termination of action potentials in excitable cardiac cells. Mutations in the hERG gene and hERG channel blockage by small molecules are associated with increased risk of fatal arrhythmias. Several drugs have been withdrawn from the market due to hERG channel-related cardiotoxicity. Moreover, as a result of its notorious ligand promiscuity, this ion channel has emerged as an important antitarget in early drug discovery and development. Surprisingly, the hERG channel blocking profile of natural compounds present in frequently consumed botanicals (i.e. dietary supplements, spices, and herbal medicinal products) is not routinely assessed. This comprehensive review will address these issues and provide a critical compilation of hERG channel data for isolated natural products and extracts over the past two decades (1996-2016). In addition, the review will provide (i) a solid basis for the molecular understanding of the physiological functions of the hERG channel, (ii) the translational potential of in vitro/in vivo results to cardiotoxicity in humans, (iii) approaches for the identification of hERG channel blockers from natural sources, (iv) future perspectives for cardiac safety guidelines and their applications within phytopharmaceuticals and dietary supplements, and (v) novel applications of hERG channel modulation (e.g. as a drug target).

hERG Channel Blocking Ipecac Alkaloids Identified by Combined In Silico - In Vitro Screening.

Human ether-a-go-go-related gene channel blocking is associated with QT interval prolongation and increased risk of potentially fatal arrhythmias. As natural products keep increasing in popularity, there is an urgent need for studies assessing human ether-a-go-go-related gene channel-related cardiotoxic risks. We selected 49 plant species based on the results of a pharmacophore-based virtual screening campaign, in parallel with a literature data survey concerning highly consumed herbal medicines with reported cardiac liabilities. Lead-like enhanced extracts were prepared, an initial in vitro screening was performed at 100 µg/mL by voltage clamp on Xenopus oocytes, and five human ether-a-go-go-related gene channel blocking extracts were identified. In accordance to the six virtually predicted alkaloids, the root extract of Carapichea ipecacuanha inhibited human ether-a-go-go-related gene channel currents by 32.5 %. A phytochemical workflow resulted in the isolation and identification of five out of the six virtually predicted alkaloids. All isolates blocked human ether-a-go-go-related gene channel currents to different extents. The major ipecac constituents emetine (1) and cephaeline (2) showed IC50 values of 21.4 and 5.3 µM, respectively, measured by whole-cell patch clamp in HEK293 cells. This is the first report on human ether-a-go-go-related gene channel blockers from C. ipecacuanha. Its roots and rhizomes are used to produce different pharmacopeial ipecac preparations that are mainly used as emetics for poisoning treatment. Our findings raise further questions regarding the safety and over-the-counter appropriateness of these herbal products.

Differential hERG ion channel activity of ultrasmall gold nanoparticles.

Understanding the mechanism of toxicity of nanomaterials remains a challenge with respect to both mechanisms involved and product regulation. Here we show toxicity of ultrasmall gold nanoparticles (AuNPs). Depending on the ligand chemistry, 1.4-nm-diameter AuNPs failed electrophysiology-based safety testing using human embryonic kidney cell line 293 cells expressing human ether-á-go-go-Related gene (hERG), a Food and Drug Administration-established drug safety test. In patch-clamp experiments, phosphine-stabilized AuNPs irreversibly blocked hERG channels, whereas thiol-stabilized AuNPs of similar size had no effect in vitro, and neither particle blocked the channel in vivo. We conclude that safety regulations may need to be reevaluated and adapted to reflect the fact that the binding modality of surface functional groups becomes a relevant parameter for the design of nanoscale bioactive compounds.

Voltage-dependent electrogenic chloride/proton exchange by endosomal CLC proteins.

Eukaryotic members of the CLC gene family function as plasma membrane chloride channels, or may provide neutralizing anion currents for V-type H(+)-ATPases that acidify compartments of the endosomal/lysosomal pathway. Loss-of-function mutations in the endosomal protein ClC-5 impair renal endocytosis and lead to kidney stones, whereas loss of function of the endosomal/lysosomal protein ClC-7 entails osteopetrosis and lysosomal storage disease. Vesicular CLCs have been thought to be Cl- channels, in particular because ClC-4 and ClC-5 mediate plasma membrane Cl- currents upon heterologous expression. Here we show that these two mainly endosomal CLC proteins instead function as electrogenic Cl-/H+ exchangers (also called antiporters), resembling the transport activity of the bacterial protein ClC-e1, the crystal structure of which has already been determined. Neutralization of a critical glutamate residue not only abolished the steep voltage-dependence of transport, but also eliminated the coupling of anion flux to proton counter-transport. ClC-4 and ClC-5 may still compensate the charge accumulation by endosomal proton pumps, but are expected to couple directly vesicular pH gradients to Cl- gradients.

CLC chloride channels and transporters.

CLC proteins are found in cells from prokaryotes to mammals and perform functions in plasma membranes and intracellular vesicles. Several genetic human diseases and mouse models underscore their broad physiological functions in mammals. These functions range from the control of excitability to transepithelial transport, endocytotic trafficking and acidification of synaptic vesicles. The recent crystallization of bacterial CLC proteins gave surprising insights into CLC Cl(-)-channel permeation and gating and provides an excellent basis for structure-function studies. Surprisingly, the CLC from Escherichia coli functions as a Cl-/H+ exchanger, thus demonstrating the thin line separating transporters and channels.

C2-Modified Sparteine Derivatives Are a New Class of Potentially Long-Acting Sodium Channel Blockers.

The lupin alkaloid sparteine is a well-known chiral diamine with a range of applications in asymmetric synthesis, as well as a blocker of voltage-gated sodium channels (VGSCs). However, there is only scarce information on the VGSC-blocking activity of sparteine derivatives where the structure of the parent alkaloid is retained. Building on the recent renewed availability of sparteine and derivatives we report herein how modification of sparteine at position 2 produces irreversible blockers of VGSCs. These compounds could be clinically envisaged as long-lasting local anesthetics.

Human Ether-à-go-go Related Gene (hERG) Channel Blocking Aporphine Alkaloids from Lotus Leaves and Their Quantitative Analysis in Dietary Weight Loss Supplements.

Blockage of the human ether-à-go-go related gene (hERG) channel can result in life-threatening ventricular tachyarrhythmia. In an in vitro screening of herbal materials for hERG blockers using an automated two-microelectrode voltage clamp assay on Xenopus oocytes, an alkaloid fraction of Nelumbo nucifera Gaertn. (lotus) leaves induced ∼50% of hERG current inhibition at 100 µg/mL. Chromatographic separation resulted in the isolation and identification of (-)-asimilobine, 1, nuciferine, 2, O-nornuciferine, 3, N-nornuciferine, 4, and liensinine, 5. In agreement with in silico predicted ligand-target interactions, 2, 3, and 4 revealed distinct in vitro hERG blockages measured in HEK293 cells with IC50 values of 2.89, 7.91, and 9.75 µM, respectively. Because lotus leaf dietary weight loss supplements are becoming increasingly popular, the identified hERG-blocking alkaloids were quantitated in five commercially available products. Results showed pronounced differences in the content of hERG-blocking alkaloids ranging up to 992 µg (2) in the daily recommended dose.

Action potential characterization of human induced pluripotent stem cell-derived cardiomyocytes using automated patch-clamp technology.

Recent progress in embryonic stem cell (ESC) and induced pluripotent stem cell (iPSC) research led to high-purity preparations of human cardiomyocytes (CMs) differentiated from these two sources-suitable for tissue regeneration, in vitro models of disease, and cardiac safety pharmacology screening. We performed a detailed characterization of the effects of nifedipine, cisapride, and tetrodotoxin (TTX) on Cor.4U(®) human iPSC-CM, using automated whole-cell patch-clamp recordings with the CytoPatch™ 2 equipment, within a complex assay combining multiple voltage-clamp and current-clamp protocols in a well-defined sequence, and quantitative analysis of several action potential (AP) parameters. We retrieved three electrical phenotypes based on AP shape: ventricular, atrial/nodal, and S-type (with ventricular-like depolarization and lack of plateau). To suppress spontaneous firing, present in many cells, we injected continuously faint hyperpolarizing currents of -10 or -20 pA. We defined quality criteria (both seal and membrane resistance over 1 GΩ), and focused our study on cells with ventricular-like AP. Nifedipine induced marked decreases in AP duration (APD): APD90 (49.8% and 40.8% of control values at 1 and 10 µM, respectively), APD50 (16.1% and 12%); cisapride 0.1 µM increased APD90 to 176.2%; and tetrodotoxin 10 µM decreased maximum slope of phase to 33.3% of control, peak depolarization potential to 76.3% of control, and shortened APD90 on average to 80.4%. These results prove feasibility of automated voltage- and current-clamp recordings on human iPSC-CM and their potential use for in-depth drug evaluation and proarrhythmic liability assessment, as well as for diagnosis and pharmacology tests for cardiac channelopathy patients.

Late cardiac sodium current can be assessed using automated patch-clamp.

The cardiac late Na (+) current is generated by a small fraction of voltage-dependent Na (+) channels that undergo a conformational change to a burst-gating mode, with repeated openings and closures during the action potential (AP) plateau. Its magnitude can be augmented by inactivation-defective mutations, myocardial ischemia, or prolonged exposure to chemical compounds leading to drug-induced (di)-long QT syndrome, and results in an increased susceptibility to cardiac arrhythmias. Using CytoPatch™ 2 automated patch-clamp equipment, we performed whole-cell recordings in HEK293 cells stably expressing human Nav1.5, and measured the late Na (+) component as average current over the last 100 ms of 300 ms depolarizing pulses to -10 mV from a holding potential of -100 mV, with a repetition frequency of 0.33 Hz. Averaged values in different steady-state experimental conditions were further corrected by the subtraction of current average during the application of tetrodotoxin (TTX) 30 µM. We show that ranolazine at 10 and 30 µM in 3 min applications reduced the late Na (+) current to 75.0 ± 2.7% (mean ± SEM, n = 17) and 58.4 ± 3.5% ( n = 18) of initial levels, respectively, while a 5 min application of veratridine 1 µM resulted in a reversible current increase to 269.1 ± 16.1% ( n = 28) of initial values. Using fluctuation analysis, we observed that ranolazine 30 µM decreased mean open probability p from 0.6 to 0.38 without modifying the number of active channels n, while veratridine 1 µM increased n 2.5-fold without changing p. In human iPSC-derived cardiomyocytes, veratridine 1 µM reversibly increased APD90 2.12 ± 0.41-fold (mean ± SEM, n = 6). This effect is attributable to inactivation removal in Nav1.5 channels, since significant inhibitory effects on hERG current were detected at higher concentrations in hERG-expressing HEK293 cells, with a 28.9 ± 6.0% inhibition (mean ± SD, n = 10) with 50 µM veratridine.       

Understanding of molecular substructures that contribute to hERG K+ channel blockade: synthesis and biological evaluation of E-4031 analogues.

Cardiotoxicity is a common side effect of a large variety of drugs that is often caused by off-target human ether-à-go-go-related gene (hERG) potassium channel blockade. In this study, we designed and synthesized a series of derivatives of the class III antiarrhythmic agent E-4031. These compounds where evaluated in a radioligand binding assay and automated patch clamp assay to establish structure-activity relationships (SAR) for their inhibition of the hERG K(+) channel. Structural modifications of E-4031 were made by altering the peripheral aromatic moieties with a series of distinct substituents. Additionally, we synthesized several derivatives with a quaternary nitrogen and modified the center of the molecule by introduction of an additional nitrogen and deletion of the carbonyl oxygen. Some modifications caused a great increase in affinity for the hERG K(+) channel, while other seemingly minor changes led to a strongly diminished affinity. Structures with quaternary amines carrying an additional aromatic moiety were found to be highly active in radioligand binding assay. A decrease in affinity was achieved by introducing an amide functionality in the central scaffold without directly interfering with the pK(a) of the essential basic amine. The knowledge gained from this study could be used in early stages of drug discovery and drug development to avoid or circumvent hERG K(+) channel blockade, thereby reducing the risk of cardiotoxicity, related arrhythmias and sudden death.

Introduction of a modular automated voltage-clamp platform and its correlation with manual human Ether-à-go-go related gene voltage-clamp data.

In investigating ion channel pharmacology, the manual patch clamp is still considered the gold standard for data quality, notwithstanding the major drawbacks of low throughput and the need for skilled operators. The automated patch clamp platform CytoPatch™ Instrument overcomes these restrictions. Its modular fully automated design makes it possible to obtain scalable throughput without the need for well-trained operators. Its chip design and perfusion system reproduces the manual patch technique, thus ensuring optimal data quality. Further, the use of stably transfected frozen cells, usable immediately after thawing, eliminates the cell quality impairment and low success rates associated with a running cell culture and renders screening costs accurately calculable. To demonstrate the applicability of this platform, 18 blinded compounds were assessed for their impact on the cardiac human Ether-à-go-go related gene K(+) channel. The IC(50) values obtained by the CytoPatch Instrument using the frozen human embryonic kidney 293 cells showed a high correlation (R(2)=0.928) with those obtained from manual patch clamp recordings with human embryonic kidney 293 cells from a running cell culture. Moreover, this correlation extended to sticky compounds such as terfenadine or astemizole. In conclusion, the CytoPatch Instrument operated with frozen cells ready to use directly after thawing provides the same high data quality known from the manual voltage clamp and has the added benefit of enhanced throughput for use in ion channel screening and safety assessment.

Determinants of anion-proton coupling in mammalian endosomal CLC proteins.

Many proteins of the CLC gene family are Cl(-) channels, whereas others, like the bacterial ecClC-1 or mammalian ClC-4 and -5, mediate Cl(-)/H(+) exchange. Mutating a "gating glutamate" (Glu-224 in ClC-4 and Glu-211 in ClC-5) converted these exchangers into anion conductances, as did the neutralization of another, intracellular "proton glutamate" in ecClC-1. We show here that neutralizing the proton glutamate of ClC-4 (Glu-281) and ClC-5 (Glu-268), but not replacing it with aspartate, histidine, or tyrosine, rather abolished Cl(-) and H(+) transport. Surface expression was unchanged by these mutations. Uncoupled Cl(-) transport could be restored in the ClC-4(E281A) and ClC-5(E268A) proton glutamate mutations by additionally neutralizing the gating glutamates, suggesting that wild type proteins transport anions only when protons are supplied through a cytoplasmic H(+) donor. Each monomeric unit of the dimeric protein was found to be able to carry out Cl(-)/H(+) exchange independently from the transport activity of the neighboring subunit. NO(3)(-) or SCN(-) transport was partially uncoupled from H(+) countertransport but still depended on the proton glutamate. Inserting proton glutamates into CLC channels altered their gating but failed to convert them into Cl(-)/H(+) exchangers. Noise analysis indicated that ClC-5 switches between silent and transporting states with an apparent unitary conductance of 0.5 picosiemens. Our results are consistent with the idea that Cl(-)/H(+) exchange of the endosomal ClC-4 and -5 proteins relies on proton delivery from an intracellular titratable residue at position 268 (numbering of ClC-5) and that the strong rectification of currents arises from the voltage-dependent proton transfer from Glu-268 to Glu-211.

[Regular prevalence surveys of hospital-acquired infections as part of the process of accreditation]. / Regelmaessige praevalensundersøgelser af hospitalsinfektioner--et led i akkrediteringsprocessen.

INTRODUCTION: All Danish hospitals are obliged to start a process of accreditation during 2009. On this background and in order to create a baseline for hospital-acquired infections (HAIs) as well as focus on infection control, we have initiated semi-annually prevalence surveys on HAIs in the North Denmark Region (RN). MATERIALS AND METHODS: Prevalence surveys were carried out in all somatic hospital wards in RN, twice in 2006 and once in 2007. The four most important HAIs were registered: urinary tract infection (UTI), pneumonia, deep surgical site infection and septicaemia. Moreover, the use of permanent and intermittent urinary tract catheters was registered. RESULTS: The overall prevalence rate in the three surveys varied between 5.2 and 7.1%. The number of patients included varied from 93.7%-98.9% of available beds. The prevalence rate for deep surgical site infections increased from the 1st to the 2nd survey, although not significantly. In the 1st survey a significantly greater number of patients with urinary tract catheters had UTIs, compared to patients without. The two later surveys only showed a trend towards increasing UTIs among patients with urinary tract catheters compared to those without. CONCLUSIONS: Repeated prevalence surveys proved to be a quick and resource-economic way of monitoring HAIs. The registration indicated a high degree of compliance among personnel. The increase in the prevalence rate of deep surgical site infections may help us find indications for further investigation by means of a time-limited incidence survey. The possible increase in UTIs among patients with urinary tract catheters may stimulate us to evaluate the indications for catheter use.

Cell activation by ligands of the toll-like receptor and interleukin-1 receptor family depends on the function of the large-conductance potassium channel MaxiK in human macrophages.

Performing patch-clamp experiments on human macrophages, we show that the K(+) channel MaxiK is activated by lipopolysaccharide, peptidoglycan, and interleukin-1. Cytokine production initiated by several Toll-like receptor (TLR) ligands and by interleukin-1 is inhibited by MaxiK blockade. This provides evidence for functional association of the MaxiK channel and TLR signaling complexes.

Lysosomal storage disease upon disruption of the neuronal chloride transport protein ClC-6.

Mammalian CLC proteins function as Cl(-) channels or as electrogenic Cl(-)/H(+) exchangers and are present in the plasma membrane and intracellular vesicles. We now show that the ClC-6 protein is almost exclusively expressed in neurons of the central and peripheral nervous systems, with a particularly high expression in dorsal root ganglia. ClC-6 colocalized with markers for late endosomes in neuronal cell bodies. The disruption of ClC-6 in mice reduced their pain sensitivity and caused moderate behavioral abnormalities. Neuronal tissues showed autofluorescence at initial axon segments. At these sites, electron microscopy revealed electron-dense storage material that caused a pathological enlargement of proximal axons. These deposits were positive for several lysosomal proteins and other marker proteins typical for neuronal ceroid lipofuscinosis (NCL), a lysosomal storage disease. However, the lysosomal pH of Clcn6(-/-) neurons appeared normal. CLCN6 is a candidate gene for mild forms of human NCL. Analysis of 75 NCL patients identified ClC-6 amino acid exchanges in two patients but failed to prove a causative role of CLCN6 in that disease.

Loss of the chloride channel ClC-7 leads to lysosomal storage disease and neurodegeneration.

ClC-7 is a chloride channel of late endosomes and lysosomes. In osteoclasts, it may cooperate with H(+)-ATPases in acidifying the resorption lacuna. In mice and man, loss of ClC-7 or the H(+)-ATPase a3 subunit causes osteopetrosis, a disease characterized by defective bone resorption. We show that ClC-7 knockout mice additionally display neurodegeneration and severe lysosomal storage disease despite unchanged lysosomal pH in cultured neurons. Rescuing their bone phenotype by transgenic expression of ClC-7 in osteoclasts moderately increased their lifespan and revealed a further progression of the central nervous system pathology. Histological analysis demonstrated an accumulation of electron-dense material in neurons, autofluorescent structures, microglial activation and astrogliosis. Like in human neuronal ceroid lipofuscinosis, there was a strong accumulation of subunit c of the mitochondrial ATP synthase and increased amounts of lysosomal enzymes. Such alterations were minor or absent in ClC-3 knockout mice, despite a massive neurodegeneration. Osteopetrotic oc/oc mice, lacking a functional H(+)-ATPase a3 subunit, showed no comparable retinal or neuronal degeneration. There are important medical implications as defects in the H(+)-ATPase and ClC-7 can underlie human osteopetrosis.