Recording ten-fold larger IKr conductances with automated patch clamping using equimolar Cs+ solutions

Background: The rapid delayed rectifier potassium current (IKr) is important for cardiac repolarization and is most often involved in drug-induced arrhythmias. However, accurately measuring this current can be challenging in human-induced pluripotent stem cell (hiPSC)-derived cardiomyocytes because of its small current density. Interestingly, the ion channel conducting IKr, hERG channel, is not only permeable to K+ ions but also to Cs+ ions when present in equimolar concentrations inside and outside of the cell. Methods: In this study, IhERG was measured from Chinese hamster ovary (CHO)-hERG cells and hiPSC-CM using either Cs+ or K+ as the charge carrier. Equimolar Cs+ has been used in the literature in manual patch-clamp experiments, and here, we apply this approach using automated patch-clamp systems. Four different (pre)clinical drugs were tested to compare their effects on Cs+- and K+-based currents. Results: Using equimolar Cs+ solutions gave rise to approximately ten-fold larger hERG conductances. Comparison of Cs+- and K+-mediated currents upon application of dofetilide, desipramine, moxifloxacin, or LUF7244 revealed many similarities in inhibition or activation properties of the drugs studied. Using equimolar Cs+ solutions gave rise to approximately ten-fold larger hERG conductances. In hiPSC-CM, the Cs+-based conductance is larger compared to the known K+-based conductance, and the Cs+ hERG conductance can be inhibited similarly to the K+-based conductance. Conclusion: Using equimolar Cs+ instead of K+ for IhERG measurements in an automated patch-clamp system gives rise to a new method by which, for example, quick scans can be performed on effects of drugs on hERG currents. This application is specifically relevant when such experiments are performed using cells which express small IKr current densities in combination with small membrane capacitances.

Inflammation-induced mitochondrial and metabolic disturbances in sensory neurons control the switch from acute to chronic pain

Pain often persists in patients with an inflammatory disease, even when inflammation has subsided. The molecular mechanisms leading to this failure in pain resolution and the transition to chronic pain are poorly understood. Mitochondrial dysfunction in sensory neurons links to chronic pain, but its role in resolution of inflammatory pain is unclear. Transient inflammation causes neuronal plasticity, called hyperalgesic priming, which impairs resolution of pain induced by a subsequent inflammatory stimulus. We identify that hyperalgesic priming in mice increases the expression of a mitochondrial protein (ATPSc-KMT) and causes mitochondrial and metabolic disturbances in sensory neurons. Inhibition of mitochondrial respiration, knockdown of ATPSCKMT expression, or supplementation of the affected metabolite is sufficient to restore resolution of inflammatory pain and prevents chronic pain development. Thus, inflammation-induced mitochondrial-dependent disturbances in sensory neurons predispose to a failure in resolution of inflammatory pain and development of chronic pain.

Leak current, even with gigaohm seals, can cause misinterpretation of stem cell-derived cardiomyocyte action potential recordings

AIMS

Human-induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) have become an essential tool to study arrhythmia mechanisms. Much of the foundational work on these cells, as well as the computational models built from the resultant data, has overlooked the contribution of seal-leak current on the immature and heterogeneous phenotype that has come to define these cells. The aim of this study is to understand the effect of seal-leak current on recordings of action potential (AP) morphology.

METHODS AND RESULTS

Action potentials were recorded in human iPSC-CMs using patch clamp and simulated using previously published mathematical models. Our in silico and in vitro studies demonstrate how seal-leak current depolarizes APs, substantially affecting their morphology, even with seal resistances (Rseal) above 1 GΩ. We show that compensation of this leak current is difficult due to challenges with obtaining accurate measures of Rseal during an experiment. Using simulation, we show that Rseal measures (i) change during an experiment, invalidating the use of pre-rupture values, and (ii) are polluted by the presence of transmembrane currents at every voltage. Finally, we posit that the background sodium current in baseline iPSC-CM models imitates the effects of seal-leak current and is increased to a level that masks the effects of seal-leak current on iPSC-CMs.

CONCLUSION

Based on these findings, we make recommendations to improve iPSC-CM AP data acquisition, interpretation, and model-building. Taking these recommendations into account will improve our understanding of iPSC-CM physiology and the descriptive ability of models built from such data.

Interplay between calcium and sarcomeres directs cardiomyocyte maturation during regeneration

Zebrafish hearts can regenerate by replacing damaged tissue with new cardiomyocytes. Although the steps leading up to the proliferation of surviving cardiomyocytes have been extensively studied, little is known about the mechanisms that control proliferation and redifferentiation to a mature state. We found that the cardiac dyad, a structure that regulates calcium handling and excitation-contraction coupling, played a key role in the redifferentiation process. A component of the cardiac dyad called leucine-rich repeat-containing 10 (Lrrc10) acted as a negative regulator of proliferation, prevented cardiomegaly, and induced redifferentiation. We found that its function was conserved in mammalian cardiomyocytes. This study highlights the importance of the underlying mechanisms required for heart regeneration and their application to the generation of fully functional cardiomyocytes.

PITX2 induction leads to impaired cardiomyocyte function in arrhythmogenic cardiomyopathy

Arrhythmogenic cardiomyopathy (ACM) is an inherited progressive disease characterized by electrophysiological and structural remodeling of the ventricles. However, the disease-causing molecular pathways, as a consequence of desmosomal mutations, are poorly understood. Here, we identified a novel missense mutation within desmoplakin in a patient clinically diagnosed with ACM. Using CRISPR-Cas9, we corrected this mutation in patient-derived human induced pluripotent stem cells (hiPSCs) and generated an independent knockin hiPSC line carrying the same mutation. Mutant cardiomyocytes displayed a decline in connexin 43, NaV1.5, and desmosomal proteins, which was accompanied by a prolonged action potential duration. Interestingly, paired-like homeodomain 2 (PITX2), a transcription factor that acts a repressor of connexin 43, NaV1.5, and desmoplakin, was induced in mutant cardiomyocytes. We validated these results in control cardiomyocytes in which PITX2 was either depleted or overexpressed. Importantly, knockdown of PITX2 in patient-derived cardiomyocytes is sufficient to restore the levels of desmoplakin, connexin 43, and NaV1.5.

A nonlinear and time-dependent leak current in the presence of calcium fluoride patch-clamp seal enhancer

Automated patch-clamp platforms are widely used and vital tools in both academia and industry to enable high-throughput studies such as drug screening. A leak current to ground occurs whenever the seal between a pipette and cell (or internal solution and cell in high-throughput machines) is not perfectly insulated from the bath (extracellular) solution. Over 1 GΩ seal resistance between pipette and bath solutions is commonly used as a quality standard for manual patch work. With automated platforms it can be difficult to obtain such a high seal resistance between the intra- and extra-cellular solutions. One suggested method to alleviate this problem is using an F ⁻ containing internal solution together with a Ca ²⁺ containing external solution — so that a CaF ₂ crystal forms when the two solutions meet which ‘plugs the holes’ to enhance the seal resistance. However, we observed an unexpected nonlinear-in-voltage and time-dependent current using these solutions on an automated patch-clamp platform. We performed manual patch-clamp experiments with the automated patch-clamp solutions, but no biological cell, and observed the same nonlinear time-dependent leak current. The current could be completely removed by washing out F ⁻ ions to leave a conventional leak current that was linear and not time-dependent. We therefore conclude fluoride ions interacting with the CaF ₂ crystal are the origin of the nonlinear time-dependent leak current. The consequences of such a nonlinear and time-dependent leak current polluting measurements should be considered carefully if it cannot be isolated and subtracted.

Development, Implementation and Testing of a Multicellular Dynamic Action Potential Clamp Simulator for Drug Cardiac Safety Assessment

As drugs can be multichannel blockers it is important to assess their cardiac safety taking into account multiple currents. In silico action potential (AP) models have been proposed for being able to integrate drugs effect on ionic currents and generate the resulting AP. However, a mathematical description of drug effects is required, which could be inaccurate. Dynamic Clamp has been proposed for drug cardiac safety assessment. In the dynamic action potential clamp (dAPC) configuration it creates an hybrid model connecting a real cell with a computer simulation. This way, drugs could be administrated directly to real cells, and effects on currents can be taken into account when generating the AP. Here we design and simulate a parallel multichannel dAPC system. The system includes the real cells overexpressing the currents of interest, the voltage clamp acquisition system, and the AP in silico model.

A nonlinear and time-dependent leak current in the presence of calcium fluoride patch-clamp seal enhancer

Automated patch-clamp platforms are widely used and vital tools in both academia and industry to enable high-throughput studies such as drug screening. A leak current to ground occurs whenever the seal between a pipette and cell (or internal solution and cell in high-throughput machines) is not perfectly insulated from the bath (extracellular) solution. Over 1 GΩ seal resistance between pipette and bath solutions is commonly used as a quality standard for manual patch work. With automated platforms it can be difficult to obtain such a high seal resistance between the intra- and extra-cellular solutions. One suggested method to alleviate this problem is using an F− containing internal solution together with a Ca2+ containing external solution — so that a CaF2 crystal forms when the two solutions meet which ‘plugs the holes’ to enhance the seal resistance. However, we observed an unexpected nonlinear-in-voltage and time-dependent current using these solutions on an automated patch-clamp platform. We performed manual patch-clamp experiments with the automated patch-clamp solutions, but no biological cell, and observed the same nonlinear time-dependent leak current. The current could be completely removed by washing out F− ions to leave a conventional leak current that was linear and not time-dependent. We therefore conclude fluoride ions interacting with the CaF2 crystal are the origin of the nonlinear time-dependent leak current. The consequences of such a nonlinear and time-dependent leak current polluting measurements should be considered carefully if it cannot be isolated and subtracted.

Accounting for variability in ion current recordings using a mathematical model of artefacts in voltage-clamp experiments

Mathematical models of ion channels, which constitute indispensable components of action potential models, are commonly constructed by fitting to whole-cell patch-clamp data. In a previous study, we fitted cell-specific models to hERG1a (Kv11.1) recordings simultaneously measured using an automated high-throughput system, and studied cell-cell variability by inspecting the resulting model parameters. However, the origin of the observed variability was not identified. Here, we study the source of variability by constructing a model that describes not just ion current dynamics, but the entire voltage-clamp experiment. The experimental artefact components of the model include: series resistance, membrane and pipette capacitance, voltage offsets, imperfect compensations made by the amplifier for these phenomena, and leak current. In this model, variability in the observations can be explained by either cell properties, measurement artefacts, or both. Remarkably, by assuming that variability arises exclusively from measurement artefacts, it is possible to explain a larger amount of the observed variability than when assuming cell-specific ion current kinetics. This assumption also leads to a smaller number of model parameters. This result suggests that most of the observed variability in patch-clamp data measured under the same conditions is caused by experimental artefacts, and hence can be compensated for in post-processing by using our model for the patch-clamp experiment. This study has implications for the question of the extent to which cell-cell variability in ion channel kinetics exists, and opens up routes for better correction of artefacts in patch-clamp data. This article is part of the theme issue 'Uncertainty quantification in cardiac and cardiovascular modelling and simulation'.

Required GK1 to Suppress Automaticity of iPSC-CMs Depends Strongly on IK1 Model Structure

Human-induced pluripotent stem cells derived cardiomyocytes (hiPSC-CMs) are a virtually endless source of human cardiomyocytes that may become a great tool for safety pharmacology; however, their electrical phenotype is immature: they show spontaneous action potentials (APs) and an unstable and depolarized resting membrane potential (RMP) because of lack of I_K1. Such immaturity hampers their application in assessing drug safety. The electronic overexpression of I_K1 (e.g., through the dynamic clamp (DC) technique) is an option to overcome this deficit. In this computational study, we aim to estimate how much I_K1 is needed to bring hiPSC-CMs to a stable and hyperpolarized RMP and which mathematical description of I_K1 is most suitable for DC experiments. We compared five mature I_K1 formulations (Bett, Dhamoon, Ishihara, O’Hara-Rudy, and ten Tusscher) with the native one (Paci), evaluating the main properties (outward peak, degree of rectification), and we quantified their effects on AP features (RMP, V˙max, APD₅₀, APD₉₀ (AP duration at 50 and 90% of repolarization), and APD₅₀/APD₉₀) after including them in the hiPSC-CM mathematical model by Paci. Then, we automatically identified the critical conductance for I_K1 ( G_{K1, critical}), the minimally required amount of I_K1 suppressing spontaneous activity. Preconditioning the cell model with depolarizing/hyperpolarizing prepulses allowed us to highlight time dependency of the I_K1 formulations. Simulations showed that inclusion of mature I_K1 formulations resulted in hyperpolarized RMP and higher V˙max, and observed G_{K1, critical} and the effect on AP duration strongly depended on I_K1 formulation. Finally, the Ishihara I_K1 led to shorter (−16.3%) and prolonged (+6.5%) APD₉₀ in response to hyperpolarizing and depolarizing prepulses, respectively, whereas other models showed negligible effects. Fine-tuning of G_K1 is an important step in DC experiments. Our computational work proposes a procedure to automatically identify how much I_K1 current is required to inject to stop the spontaneous activity and suggests the use of the Ishihara I_K1 model to perform DC experiments in hiPSC-CMs.

Genetic variation in GNB5 causes bradycardia by augmenting the cholinergic response via increased acetylcholine-activated potassium current (I K,ACh)

Mutations in GNB5, encoding the G-protein β5 subunit (Gβ5), have recently been linked to a multisystem disorder that includes severe bradycardia. Here, we investigated the mechanism underlying bradycardia caused by the recessive p.S81L Gβ5 variant. Using CRISPR/Cas9-based targeting, we generated an isogenic series of human induced pluripotent stem cell (hiPSC) lines that were either wild type, heterozygous or homozygous for the GNB5 p.S81L variant. These were differentiated into cardiomyocytes (hiPSC-CMs) that robustly expressed the acetylcholine-activated potassium channel [I(KACh); also known as I_K,ACh]. Baseline electrophysiological properties of the lines did not differ. Upon application of carbachol (CCh), homozygous p.S81L hiPSC-CMs displayed an increased acetylcholine-activated potassium current (I _K,ACh) density and a more pronounced decrease of spontaneous activity as compared to wild-type and heterozygous p.S81L hiPSC-CMs, explaining the bradycardia in homozygous carriers. Application of the specific I(KACh) blocker XEN-R0703 resulted in near-complete reversal of the phenotype. Our results provide mechanistic insights and proof of principle for potential therapy in patients carrying GNB5 mutations.This article has an associated First Person interview with the first author of the paper.

The influence of hERG1a and hERG1b isoforms on drug safety screening in iPSC-CMs

The human Ether-à-go-go Related Gene (hERG) encodes the pore forming subunit of the channel that conducts the rapid delayed rectifier potassium current I_Kr. I_Kr drives repolarization in the heart and when I_Kr is dysfunctional, cardiac repolarization delays, the QT interval on the electrocardiogram (ECG) prolongs and the risk of developing lethal arrhythmias such as Torsade de Pointes (TdP) increases. TdP risk is incorporated in drug safety screening for cardiotoxicity where hERG is the main target since the I_Kr channels appear highly sensitive to blockage. hERG block is also included as an important read-out in the Comprehensive in Vitro Proarrhythmia Assay (CiPA) initiative which aims to combine in vitro and in silico experiments on induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) to screen for cardiotoxicity. However, the hERG channel has some unique features to consider for drug safety screening, which we will discuss in this study. The hERG channel consists of different isoforms, hERG1a and hERG1b, which individually influence the kinetics of the channel and the drug response in the human heart and in iPSC-CMs. hERG1b is often underappreciated in iPSC-CM studies, drug screening assays and in silico models, and the fact that its contribution might substantially differ between iPSC-CM and healthy but also diseased human heart, adds to this problem. In this study we show that the activation kinetics in iPSC-CMs resemble hERG1b kinetics using Cs⁺ as a charge carrier. Not including hERG1b in drug safety testing might underestimate the actual role of hERG1b in repolarization and drug response, and might lead to inappropriate conclusions. We stress to focus more on including hERG1b in drug safety testing concerning I_Kr.

Flotillins in the intercalated disc are potential modulators of cardiac excitability

BACKGROUND

The intercalated disc (ID) is important for cardiac remodeling and has become a subject of intensive research efforts. However, as yet the composition of the ID has still not been conclusively resolved and the role of many proteins identified in the ID, like Flotillin-2, is often unknown. The Flotillin proteins are known to be involved in the stabilization of cadherins and desmosomes in the epidermis and upon cancer development. However, their role in the heart has so far not been investigated. Therefore, in this study, we aimed at identifying the role of Flotillin-1 and Flotillin-2 in the cardiac ID.

METHODS

Location of Flotillins in human and murine cardiac tissue was evaluated by fluorescent immunolabeling and co-immunoprecipitation. In addition, the effect of Flotillin knockout (KO) on proteins of the ID and in electrical excitation and conduction was investigated in cardiac samples of wildtype (WT), Flotillin-1 KO, Flotilin-2 KO and Flotilin-1/2 double KO mice. Consequences of Flotillin knockdown (KD) on cardiac function were studied (patch clamp and Multi Electrode Array (MEA)) in neonatal rat cardiomyocytes (NRCMs) transfected with siRNAs against Flotillin-1 and/or Flotillin-2.

RESULTS

First, we confirmed presence in the ID and mutual binding of Flotillin-1 and Flotillin-2 in murine and human cardiac tissue. Flotillin KO mice did not show cardiac fibrosis, nor hypertrophy or changes in expression of the desmosomal ID proteins. However, protein expression of the cardiac sodium channel Na_V1.5 was significantly decreased in Flotillin-1 and Flotillin-1/2 KO mice compared to WT mice. In addition, sodium current density showed a significant decrease upon Flotillin-1/2 KD in NRCMs as compared to scrambled siRNA-transfected NRCMs. MEA recordings of Flotillin-2 KD NRCM cultures showed a significantly decreased spike amplitude and a tendency of a reduced spike slope when compared to control and scrambled siRNA-transfected cultures.

CONCLUSIONS

In this study, we demonstrate the presence of Flotillin-1, in addition to Flotillin-2 in the cardiac ID. Our findings indicate a modulatory role of Flotillins on Na_V1.5 expression at the ID, with potential consequences for cardiac excitation.

Optogenetic sensors in the zebrafish heart: a novel in vivo electrophysiological tool to study cardiac arrhythmogenesis

Cardiac arrhythmias are among the most challenging human disorders to diagnose and treat due to their complex underlying pathophysiology. Suitable experimental animal models are needed to study the mechanisms causative for cardiac arrhythmogenesis. To enable in vivo analysis of cardiac cellular electrophysiology with a high spatial and temporal resolution, we generated and carefully validated two zebrafish models, one expressing an optogenetic voltage indicator (chimeric VSFP-butterfly CY) and the other a genetically encoded calcium indicator (GCaMP6f) in the heart.

Methods: High-speed epifluorescence microscopy was used to image chimeric VSFP-butterfly CY and GCaMP6f in the embryonic zebrafish heart, providing information about the spatiotemporal patterning of electrical activation, action potential configuration and intracellular Ca²⁺ dynamics. Plotting VSFP or GCaMP6f signals on a line along the myocardial wall over time facilitated the visualization and analysis of electrical impulse propagation throughout the heart. Administration of drugs targeting the sympathetic nervous system or cardiac ion channels was used to validate sensitivity and kinetics of both zebrafish sensor lines. Using the same microscope setup, we imaged transparent juvenile casper fish expressing GCaMP6f, demonstrating the feasibility of imaging cardiac optogenetic sensors at later stages of development.

Results: Isoproterenol slightly increased heart rate, diastolic Ca²⁺ levels and Ca²⁺ transient amplitudes, whereas propranolol caused a profound decrease in heart rate and Ca²⁺ transient parameters in VSFP-Butterfly and GCaMP6f embryonic fish. Ik_r blocker E-4031 decreased heart rate and increased action potential duration in VSFP-Butterfly fish. I_Ca,L blocker nifedipine caused total blockade of Ca²⁺ transients in GCaMP6f fish and a reduced heart rate, altered ventricular action potential duration and disrupted atrial-ventricular electrical conduction in VSFP-Butterfly fish. Imaging of juvenile animals demonstrated the possibility of employing an older zebrafish model for in vivo cardiac electrophysiology studies. We observed differences in atrial and ventricular Ca²⁺ recovery dynamics between 3 dpf and 14 dpf casper fish, but not in Ca²⁺ upstroke dynamics.

Conclusion: By introducing the optogenetic sensors chimeric VSFP-butterfly CY and GCaMP6f into the zebrafish we successfully generated an in vivo cellular electrophysiological readout tool for the zebrafish heart. Complementary use of both sensor lines demonstrated the ability to study heart rate, cardiac action potential configuration, spatiotemporal patterning of electrical activation and intracellular Ca²⁺ homeostasis in embryonic zebrafish. In addition, we demonstrated the first successful use of an optogenetic sensor to study cardiac function in older zebrafish. These models present a promising new research tool to study the underlying mechanisms of cardiac arrhythmogenesis.

Sinusoidal voltage protocols for rapid characterisation of ion channel kinetics

Key points

Ion current kinetics are commonly represented by current–voltage relationships, time constant–voltage relationships and subsequently mathematical models fitted to these. These experiments take substantial time, which means they are rarely performed in the same cell.

Rather than traditional square‐wave voltage clamps, we fitted a model to the current evoked by a novel sum‐of‐sinusoids voltage clamp that was only 8 s long.

Short protocols that can be performed multiple times within a single cell will offer many new opportunities to measure how ion current kinetics are affected by changing conditions.

The new model predicts the current under traditional square‐wave protocols well, with better predictions of underlying currents than literature models. The current under a novel physiologically relevant series of action potential clamps is predicted extremely well.

The short sinusoidal protocols allow a model to be fully fitted to individual cells, allowing us to examine cell–cell variability in current kinetics for the first time.

Abstract

Understanding the roles of ion currents is crucial to predict the action of pharmaceuticals and mutations in different scenarios, and thereby to guide clinical interventions in the heart, brain and other electrophysiological systems. Our ability to predict how ion currents contribute to cellular electrophysiology is in turn critically dependent on our characterisation of ion channel kinetics – the voltage‐dependent rates of transition between open, closed and inactivated channel states. We present a new method for rapidly exploring and characterising ion channel kinetics, applying it to the hERG potassium channel as an example, with the aim of generating a quantitatively predictive representation of the ion current. We fitted a mathematical model to currents evoked by a novel 8 second sinusoidal voltage clamp in CHO cells overexpressing hERG1a. The model was then used to predict over 5 minutes of recordings in the same cell in response to further protocols: a series of traditional square step voltage clamps, and also a novel voltage clamp comprising a collection of physiologically relevant action potentials. We demonstrate that we can make predictive cell‐specific models that outperform the use of averaged data from a number of different cells, and thereby examine which changes in gating are responsible for cell–cell variability in current kinetics. Our technique allows rapid collection of consistent and high quality data, from single cells, and produces more predictive mathematical ion channel models than traditional approaches.

Ion current kinetics are commonly represented by current–voltage relationships, time constant–voltage relationships and subsequently mathematical models fitted to these. These experiments take substantial time, which means they are rarely performed in the same cell.

Rather than traditional square‐wave voltage clamps, we fitted a model to the current evoked by a novel sum‐of‐sinusoids voltage clamp that was only 8 s long.

Short protocols that can be performed multiple times within a single cell will offer many new opportunities to measure how ion current kinetics are affected by changing conditions.

The new model predicts the current under traditional square‐wave protocols well, with better predictions of underlying currents than literature models. The current under a novel physiologically relevant series of action potential clamps is predicted extremely well.

The short sinusoidal protocols allow a model to be fully fitted to individual cells, allowing us to examine cell–cell variability in current kinetics for the first time.

A Hybrid Model for Safety Pharmacology on an Automated Patch Clamp Platform: Using Dynamic Clamp to Join iPSC-Derived Cardiomyocytes and Simulations of Ik1 Ion Channels in Real-Time

An important aspect of the Comprehensive In Vitro Proarrhythmia Assay (CiPA) proposal is the use of human stem cell-derived cardiomyocytes and the confirmation of their predictive power in drug safety assays. The benefits of this cell source are clear; drugs can be tested in vitro on human cardiomyocytes, with patient-specific genotypes if needed, and differentiation efficiencies are generally excellent, resulting in a virtually limitless supply of cardiomyocytes. There are, however, several challenges that will have to be surmounted before successful establishment of hSC-CMs as an all-round predictive model for drug safety assays. An important factor is the relative electrophysiological immaturity of hSC-CMs, which limits arrhythmic responses to unsafe drugs that are pro-arrhythmic in humans. Potentially, immaturity may be improved functionally by creation of hybrid models, in which the dynamic clamp technique joins simulations of lacking cardiac ion channels (e.g., I_K1) with hSC-CMs in real-time during patch clamp experiments. This approach has been used successfully in manual patch clamp experiments, but throughput is low. In this study, we combined dynamic clamp with automated patch clamp of iPSC-CMs in current clamp mode, and demonstrate that I_K1 conductance can be added to iPSC-CMs on an automated patch clamp platform, resulting in an improved electrophysiological maturity.

The immature electrophysiological phenotype of iPSC-CMs still hampers in vitro drug screening: Special focus on IK1

Preclinical drug screens are not based on human physiology, possibly complicating predictions on cardiotoxicity. Drug screening can be humanised with in vitro assays using human induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs). However, in contrast to adult ventricular cardiomyocytes, iPSC-CMs beat spontaneously due to presence of the pacemaking current I_f and reduced densities of the hyperpolarising current I_K1. In adult cardiomyocytes, I_K1 finalises repolarisation by stabilising the resting membrane potential while also maintaining excitability. The reduced I_K1 density contributes to proarrhythmic traits in iPSC-CMs, which leads to an electrophysiological phenotype that might bias drug responses. The proarrhythmic traits can be suppressed by increasing I_K1 in a balanced manner. We systematically evaluated all studies that report strategies to mature iPSC-CMs and found that only few studies report I_K1 current densities. Furthermore, these studies did not succeed in establishing sufficient I_K1 levels as they either added too little or too much I_K1. We conclude that reduced densities of I_K1 remain a major flaw in iPSC-CMs, which hampers their use for in vitro drug screening.

Cardiac optogenetics: using light to monitor cardiac physiology

Our current understanding of cardiac excitation and its coupling to contraction is largely based on ex vivo studies utilising fluorescent organic dyes to assess cardiac action potentials and signal transduction. Recent advances in optogenetic sensors open exciting new possibilities for cardiac research and allow us to answer research questions that cannot be addressed using the classic organic dyes. Especially thrilling is the possibility to use optogenetic sensors to record parameters of cardiac excitation and contraction in vivo. In addition, optogenetics provide a high spatial resolution, as sensors can be coupled to motifs and targeted to specific cell types and subcellular domains of the heart. In this review, we will give a comprehensive overview of relevant optogenetic sensors, how they can be utilised in cardiac research and how they have been applied in cardiac research up to now.

Perpetuation of torsade de pointes in heterogeneous hearts: competing foci or re-entry?

KEY POINTS

The underlying mechanism of torsade de pointes (TdP) remains of debate: perpetuation may be due to (1) focal activity or (2) re-entrant activity. The onset of TdP correlates with action potential heterogeneities in different regions of the heart. We studied the mechanism of perpetuation of TdP in silico using a 2D model of human cardiac tissue and an anatomically accurate model of the ventricles of the human heart. We found that the mechanism of perpetuation TdP depends on the degree of heterogeneity. If the degree of heterogeneity is large, focal activity alone can sustain a TdP, otherwise re-entrant activity emerges. This result can help to understand the relationship between the mechanisms of TdP and tissue properties and may help in developing new drugs against it.

ABSTRACT

Torsade de pointes (TdP) can be the consequence of cardiac remodelling, drug effects or a combination of both. The mechanism underlying TdP is unclear, and may involve triggered focal activity or re-entry. Recent work by our group has indicated that both cases may exist, i.e. TdPs induced in the chronic atrioventricular block (CAVB) dog model may have a focal origin or are due to re-entry. Also it was found that heterogeneities might play an important role. In the current study we have used computational modelling to further investigate the mechanisms involved in TdP initiation and perpetuation, especially in the CAVB dog model, by the addition of heterogeneities with reduced repolarization reserve in comparison with the surrounding tissue. For this, the TNNP computer model was used for computations. We demonstrated in 2D and 3D simulations that ECGs with the typical TdP morphology can be caused by both multiple competing foci and re-entry circuits as a result of introduction of heterogeneities, depending on whether the heterogeneities have a large or a smaller reduced repolarization reserve in comparison with the surrounding tissue. Large heterogeneities can produce ectopic TdP, while smaller heterogeneities will produce re-entry-type TdP.

Uncertainty and variability in models of the cardiac action potential: Can we build trustworthy models?

Cardiac electrophysiology models have been developed for over 50 years, and now include detailed descriptions of individual ion currents and sub-cellular calcium handling. It is commonly accepted that there are many uncertainties in these systems, with quantities such as ion channel kinetics or expression levels being difficult to measure or variable between samples. Until recently, the original approach of describing model parameters using single values has been retained, and consequently the majority of mathematical models in use today provide point predictions, with no associated uncertainty.

In recent years, statistical techniques have been developed and applied in many scientific areas to capture uncertainties in the quantities that determine model behaviour, and to provide a distribution of predictions which accounts for this uncertainty. In this paper we discuss this concept, which is termed uncertainty quantification, and consider how it might be applied to cardiac electrophysiology models.

We present two case studies in which probability distributions, instead of individual numbers, are inferred from data to describe quantities such as maximal current densities. Then we show how these probabilistic representations of model parameters enable probabilities to be placed on predicted behaviours. We demonstrate how changes in these probability distributions across data sets offer insight into which currents cause beat-to-beat variability in canine APs. We conclude with a discussion of the challenges that this approach entails, and how it provides opportunities to improve our understanding of electrophysiology.

•

Uncertainty and variability in action potential models can be quantified.

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A probabilistic method for inferring maximal current densities is developed and applied.

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We use this to infer the currents responsible for canine beat-to-beat variability.

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Emulation of mathematical models provides rich information at low computational cost.

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The importance of considering uncertainty and variability in future is discussed.

Sensing Cardiac Electrical Activity With a Cardiac Myocyte--Targeted Optogenetic Voltage Indicator

RATIONALE

Monitoring and controlling cardiac myocyte activity with optogenetic tools offer exciting possibilities for fundamental and translational cardiovascular research. Genetically encoded voltage indicators may be particularly attractive for minimal invasive and repeated assessments of cardiac excitation from the cellular to the whole heart level.

OBJECTIVE

To test the hypothesis that cardiac myocyte-targeted voltage-sensitive fluorescence protein 2.3 (VSFP2.3) can be exploited as optogenetic tool for the monitoring of electric activity in isolated cardiac myocytes and the whole heart as well as function and maturity in induced pluripotent stem cell-derived cardiac myocytes.

METHODS AND RESULTS

We first generated mice with cardiac myocyte-restricted expression of VSFP2.3 and demonstrated distinct localization of VSFP2.3 at the t-tubulus/junctional sarcoplasmic reticulum microdomain without any signs for associated pathologies (assessed by echocardiography, RNA-sequencing, and patch clamping). Optically recorded VSFP2.3 signals correlated well with membrane voltage measured simultaneously by patch clamping. The use of VSFP2.3 for human action potential recordings was confirmed by simulation of immature and mature action potentials in murine VSFP2.3 cardiac myocytes. Optical cardiograms could be monitored in whole hearts ex vivo and minimally invasively in vivo via fiber optics at physiological heart rate (10 Hz) and under pacing-induced arrhythmia. Finally, we reprogrammed tail-tip fibroblasts from transgenic mice and used the VSFP2.3 sensor for benchmarking functional and structural maturation in induced pluripotent stem cell-derived cardiac myocytes.

CONCLUSIONS

We introduce a novel transgenic voltage-sensor model as a new method in cardiovascular research and provide proof of concept for its use in optogenetic sensing of physiological and pathological excitation in mature and immature cardiac myocytes in vitro and in vivo.

Arrhythmogenic Remodeling in Murine Models of Deoxycorticosterone Acetate-Salt-Induced and 5/6-Subtotal Nephrectomy-Salt-Induced Cardiorenal Disease

BACKGROUND

Renal failure is associated with adverse cardiac remodeling and sudden cardiac death. The mechanism leading to enhanced arrhythmogenicity in the cardiorenal syndrome is unclear. The aim of this study was to characterize electrophysiological and tissue alterations correlated with enhanced arrhythmogenicity in two distinct mouse models of renal failure.

METHODS

Thirty-week-old 129Sv mice received a high-salt diet and deoxycorticosterone acetate (DOCA) for 8 weeks, followed by an additional period of high-salt diet for 27 weeks (DOCA-salt aged model). Adult CD-1 mice were submitted to 5/6-subtotal nephrectomy (SNx) and treated for 11 weeks with a high-salt diet (SNx-salt adult model). Vulnerability to arrhythmia as well as conduction velocities (CVs) of the hearts were determined ex vivo with epicardial mapping. Subsequently, the hearts were characterized for connexin 43 (Cx43) and fibrosis.

RESULTS

DOCA-salt and SNx-salt mice developed renal dysfunction characterized by albuminuria. Heart, lung and kidney weights were increased in DOCA-salt mice. Both DOCA-salt and SNx-salt mice were highly susceptible to ventricular arrhythmias. DOCA-salt mice had a significant decrease in both longitudinal and transversal CV in the left ventricle. Histological analysis revealed a significant reduction in Cx43 expression as well as an increase in interstitial fibrosis in both DOCA-salt and SNx-salt mice.

CONCLUSION

DOCA-salt and SNx-salt treatment induced renal dysfunction, which resulted in structural and electrical cardiac remodeling and enhanced arrhythmogenicity. The reduced Cx43 expression and increased fibrosis levels in these hearts are likely candidates for the formation of the arrhythmogenic substrate.

Experimental Mapping of the Canine KCNJ2 and KCNJ12 Gene Structures and Functional Analysis of the Canine K(IR)2.2 ion Channel

For many model organisms traditionally in use for cardiac electrophysiological studies, characterization of ion channel genes is lacking. We focused here on two genes encoding the inward rectifier current, KCNJ2 and KCNJ12, in the dog heart. A combination of RT-PCR, 5′-RACE, and 3′-RACE demonstrated the status of KCNJ2 as a two exon gene. The complete open reading frame (ORF) was located on the second exon. One transcription initiation site was mapped. Four differential transcription termination sites were found downstream of two consensus polyadenylation signals. The canine KCNJ12 gene was found to consist of three exons, with its ORF located on the third exon. One transcription initiation and one termination site were found. No alternative splicing was observed in right ventricle or brain cortex. The gene structure of canine KCNJ2 and KCNJ12 was conserved amongst other vertebrates, while current GenBank gene annotation was determined as incomplete. In silico translation of KCN12 revealed a non-conserved glycine rich stretch located near the carboxy-terminus of the K_IR2.2 protein. However, no differences were observed when comparing dog with human K_IR2.2 protein upon ectopic expression in COS-7 or HEK293 cells with respect to subcellular localization or electrophysiological properties.

Drug-Induced Torsade de Pointes Arrhythmias in the Chronic AV Block Dog Are Perpetuated by Focal Activity

Background—

The electrically remodeled canine heart after chronic AV block (CAVB) has a high susceptibility for drug-induced torsade de pointes (TdP) arrhythmias. Although focal mechanisms have been considered for initiation, there is still controversy about whether reentry is the dominant mechanism for perpetuation of TdP. In this animal model with known nonuniform prolongation of repolarization, the mechanism of perpetuation of TdP arrhythmia was explored.

Methods and Results—

Seventeen TdP-sensitive CAVB and 10 sinus rhythm (SR) dogs were studied. In 6 animals, 66 needle electrodes were evenly distributed transmurally to record 240 unipolar local electrograms simultaneously. Activation times and activation recovery intervals were determined before and during ibutilide-induced TdP. In 12 CAVB and 9 SR dogs, left ventricular (LV) and right ventricular (RV) epicardial electrograms were recorded with a 208-point multiterminal grid electrode allowing conduction velocity (CV) and ventricular effective refractory period (VERP) measurements. Biopsy specimens were processed for connexin43 (Cx43) expression and collagen content. Ventricular myocytes were isolated to determine sodium current ( I Na ) density and cell dimensions. Computer simulations were used to assess the effects of changes therein. In CAVB, VERP and ARI were increased, whereas CV was unaltered in LV. Transversal but not longitudinal CV was increased in RV. I Na was reduced by 37% in LV but unaltered in RV. LV and RV cell size were increased, but collagen and Cx43 content remained unchanged. Simulations showed increase in CV of RV as a consequence of increased cell size at normal I Na . Ibutilide increased ARI, ERP, and maximal transmural dispersion of ERP (45±25 to 120±65 ms; P <0.05). Twenty-eight of 47 episodes of self-terminating TdP (43±72 beats) were analyzed. The majority (>90%) of beats were focal; reentry was observed only occasionally.

Conclusions—

Focal activity is the dominant mechanism involved in perpetuation of ibutilide-induced TdP in CAVB dogs based on detailed 3D mapping. This conclusion is in line with unaltered conduction and documented increase in VERP.

Carvedilol inhibits Kir2.3 channels by interference with PIP₂-channel interaction

Carvedilol, a β- and α-adrenoceptor blocker, is used to treat congestive heart failure, mild to moderate hypertension, and myocardial infarction. It has been proposed to block K(ATP) channels by binding to the bundle crossing region at a domain including cysteine at position 166, and thereby plugging the pore region. However, carvedilol was reported not to affect Kir2.1 channels, which lack 166 Cys. Here, we demonstrate that carvedilol inhibits Kir2.3 carried current by an alternative mechanism. Carvedilol inhibited Kir2.3 channels with at least 100 fold higher potency (IC(50)=0.49 μM) compared to that for Kir2.1 (IC(50)>50 μM). Kir2.3 channel inhibition was concentration-dependent and voltage-independent. Increasing Kir2.3 channel affinity for PIP(2), by a I213L point mutation, decreased the inhibitory effect of carvedilol more than twentyfold (IC(50)=11.1 μM). In the presence of exogenous PIP(2), Kir2.3 channel inhibition by carvedilol was strongly reduced (80 vs. 2% current inhibition). These results suggest that carvedilol, as other cationic amphiphilic drugs, inhibits Kir2.3 channels by interfering with the PIP(2)-channel interaction.

Foetal and adult cardiomyocyte progenitor cells have different developmental potential

In the past years, cardiovascular progenitor cells have been isolated from the human heart and characterized. Up to date, no studies have been reported in which the developmental potential of foetal and adult cardiovascular progenitors was tested simultaneously. However, intrinsic differences will likely affect interpretations regarding progenitor cell potential and application for regenerative medicine. Here we report a direct comparison between human foetal and adult heart-derived cardiomyocyte progenitor cells (CMPCs). We show that foetal and adult CMPCs have distinct preferences to differentiate into mesodermal lineages. Under pro-angiogenic conditions, foetal CMPCs form more endothelial but less smooth muscle cells than adult CMPCs. Foetal CMPCs can also develop towards adipocytes, whereas neither foetal nor adult CMPCs show significant osteogenic differentiation. Interestingly, although both cell types differentiate into heart muscle cells, adult CMPCs give rise to electrophysiologically more mature cardiomyocytes than foetal CMPCs. Taken together, foetal CMPCs are suitable for molecular cell biology and developmental studies. The potential of adult CMPCs to form mature cardiomyocytes and smooth muscle cells may be essential for cardiac repair after transplantation into the injured heart.

Rewiring the heart: stem cell therapy to restore normal cardiac excitability and conduction

The regenerative capacity of the mammalian heart is insufficient to recover from myocardial infarction. Stem cells are currently considered as a promising and valuable tool to replace the, often large, loss of contractile tissue. One of the bottlenecks hampering fast clinical application is the large amount of cells required to replace a single damaged region combined with an appropriate strategy to succeed in homogeneous repair. A second class of major cardiac disorders for which stem cell therapy might be fruitful and would require less cells for repair, are chronic rhythm disorders. In this area, most research has been focused on stem-cell based biological pacemakers, but increasing amounts of data on AV nodal repair appear in literature. Both therapies, in principle, could eventually replace current instrumentation with electronic pacemakers. Finally, an emerging field of interest explores transplantation of stem cells expressing specific ion channels aiming at suppression of focal arrhythmias, providing an alternative strategy for surgical and catheter-mediated ablation. Since in this second class of applications the number of transplanted cells required may be relatively low, effective clinical therapy may be within close range. Here, we will review recent achievements in the fields of stem-cell based biological pacemakers, AV nodal repair and biological ablation.

Human cardiomyocyte progenitor cell-derived cardiomyocytes display a maturated electrical phenotype

Cardiomyocyte progenitor cells (CMPCs) can be isolated from the human heart and differentiated into cardiomyocytes in vitro. A comprehensive assessment of their electrical phenotype upon differentiation is essential to predict potential future applications of this cell source. CMPCs isolated from human fetal heart were differentiated in vitro and examined using immunohistochemistry, Western blotting, RT-PCR, voltage clamp and current clamp techniques. Differentiated cultures presented up to 95% alpha-actinin positive cardiomyocytes. Adherens junction and desmosomal proteins beta-catenin, N-cadherin, desmin and plakophilin2 were upregulated. Expression levels of cardiac connexins were not affected by differentiation, however Cx43 phosphorylation was increased upon differentiation, accompanied by translocation of connexins to the cell border. RT-PCR analysis demonstrated upregulation of all major cardiac ion channel constituents during differentiation. Patch clamp experiments showed that cardiomyocytes had a stable resting membrane potential of -73.4+/-1.8 mV. Infusion of 1 mM BaCl(2) resulted in depolarization to -59.9+/-2.8 mV, indicating I(K1) channel activity. Subsequent voltage clamp experiments confirmed presence of near mature I(Na), I(Ca,L) and I(K1) current densities. Infusion of the I(Kr) blocker Almokalant caused prolongation of the action potential by 40%. Differentiated monolayers were not spontaneously contracting in the absence of serum, but responded to field stimulation, displaying adult ventricular-like action potentials. Human fetal CMPC-derived cardiomyocytes have a homogenous and rather mature electrical phenotype that benefits to in vitro physiology and pharmacology. In the context of cardiac repair, their properties may translate into a reduced pro-arrhythmic risk and enhanced electrical integration upon transplantation.

Connexin43 repression following epithelium-to-mesenchyme transition in embryonal carcinoma cells requires Snail1 transcription factor

Embryonic stem (ES) cells and embryonal carcinoma (EC) cells express high amounts of functional connexin43 (Cx43). During mesoderm formation and subsequent cardiac differentiation, Cx43 is initially down-regulated but is up-regulated again as the emerging cardiomyocytes mature. In this study, we investigated the regulation of Cx43 expression during early phases of differentiation in F9 and P19 EC cells. We found a striking inverse correlation between the expression of Cx43 and that of the transcriptional repressor Snail1. No clear relationship was found with Smad-interacting-protein1 (SIP1), another transcription factor inducing epithelium-to-mesenchyme transition (EMT). Promoter-reporter assays indicated Cx43 repression at the promoter level by ectopically expressed Snail1. To establish whether the Cx43 down-regulation depends on endogenous Snail1, MES-1 cells, differentiated derivatives of P19 EC, were stably transfected by an siRNA construct silencing Snail1 expression. This resulted in a mesenchyme-to-epithelium transition, which was accompanied by increased levels of Cx43 mRNA and protein and enhanced metabolic and electrical coupling. We conclude that Snail1-mediated EMT results in a Cx43 repression.

Beta-, Not Alpha-Adrenergic Stimulation Enhances Conduction Velocity in Cultures of Neonatal Cardiomyocytes

BACKGROUND

During both cardiac maturation and myopathy, elevated levels of circulating catecholamines coincide with alterations in impulse propagation. An in vitro model of cultured cardiomyocytes was used to study the effects of adrenergic stimulation on the conduction characteristics of immature heart cells.

METHODS AND RESULTS

Neonatal rat cardiomyocytes were cultured on preparations designed to measure conduction velocity (CV). CV was measured on the same preparation twice at t=0 and at t=24 h. Under control conditions (n=7), CV at t=0 (30.9+/-1.9 cm/s) and t=24 (32.4+/-4.4 cm/s) was similar (p=0.70). Immunohistochemistry revealed expression of the gap junction proteins connexin (Cx) 40, Cx43 and Cx45, with Cx43 being highly predominant. Stimulation for 24 h with the beta-adrenergic agonist isoproterenol (ISO) significantly increased CV from 28.0 +/-2.0 cm/s at t=0 to 34.8+/-2.2 cm/s at t=24 (p=0.002, n=5). Microelectrode recordings showed a faster upstroke of the action potential (AP) of ISO-treated cells. Reverse transcribed-polymerase chain reactions (RT-PCR) showed that ISO increased expression of SCN5A and alpha(1c) (alpha-subunit of the cardiac sodium and L-type calcium channel, respectively). Stimulation of cells with ISO did not induce alterations in distribution or expression of Cx40, Cx43 and Cx45 (both mRNA and protein), but slightly increased the phosphorylation of Cx43. Stimulation for 24 h with the alpha-adrenergic agonist phenylephrine did neither affect CV nor the expression of the connexin isoforms, SCN5A and alpha(1c).

CONCLUSIONS

Alpha- and beta-adrenergic stimulation differently affect propagation of the electric impulse, which is primarily not caused by a differential effect on intercellular coupling. RT-PCR analysis and an enhanced AP upstroke velocity indicate a higher functional expression level of alpha(1c) and SCN5A in beta-adrenergic stimulated cells, which may explain the observed increase in CV.

Cloning, embryonic expression, and functional characterization of two novel connexins from Xenopus laevis

Vertebrate gap junctions are constituted of connexin (Cx) proteins. In Xenopus laevis, only seven different Cxs have been described so far. Here, we identify two new Cxs from X. laevis. Cx28.6 displays > 60% amino acid identity with human Cx25, Cx29 displays strong homology with mouse Cx26 and Cx30. Cx29 is expressed throughout embryonic development. Cx28.6 mRNA is only transiently found from stage 22 to 26 of development. While no Cx28.6 expression could be detected by whole mount in situ hybridization, expression of Cx29 was found in the developing endoderm, lateral mesoderm, liver anlage, pronephros, and proctodeum. Ectopic expression of Cx28.6 failed to produce functional gap-junctions. In contrast, ectopic expression of full-length Cx29 in HEK293 and COS-7 cells resulted in the formation of gap junction-like structures at the cell-cell interfaces. Ectopic expression of Cx29 in communication deficient N2A cell pairs led to functional electrical coupling.

Pro-arrhythmogenic potential of immature cardiomyocytes is triggered by low coupling and cluster size

OBJECTIVE

Cell transplantation strategies to regenerate compromised myocardium take advance of in vitro generated cardiomyocytes. Common in those immature myocytes is spontaneous impulse formation and a restricted ability to establish proper electrical interaction. Spontaneous impulse formation and impaired cell-to-cell coupling have been shown to be arrhythmogenic. To investigate whether these features harbour a pro-arrhyhmogenic potential for cell transplantation, a co-culture of spontaneously active neonatal rat cardiomyocytes (NRC) and quiescent adult dog cardiomyocytes (ADC) was used.

METHODS

ADCs and NRCs were isolated and cultured on laminin-coated substrates. Connexin43, N-cadherin and alpha-actinin expression was evaluated with immunohistochemistry. Intercellular coupling was measured in cell pairs using the dual voltage clamp technique and fluorescent dye injection.

RESULTS

One day after isolation, NRCs were beating spontaneously, while ADCs remained quiescent in monoculture. ADC resting membrane potential was -80.3+/-0.2 mV (mean+/-SEM, N=24) and did not change significantly over time. NRCs had a maximal diastolic potential of -65.0+/-2.8 mV (N=4). After one day of co-culture, pseudopodia-like extensions developed at the former intercalated discs of ADCs, contacting the NRCs. Only ADCs that contacted three or more NRCs started to beat in synchrony. Expression of connexin43 and N-cadherin indicated presence of electrical and mechanical junctions at the interface between the two cell-types. Transfer of Lucifer Yellow demonstrated junctional permeability between ADCs and NRCs. Junctional conductance between ADC-ADC (31.9+/-5.1 nS, N=10) and NRC-NRC (35.0+/-9.6 nS, N=6) pairs was significantly higher compared to ADC-NRC pairs (9.7+/-2.9 nS, N=8). Gap-junctional blockade with halothane reversibly abolished NRC-triggered beating of ADCs. Computer simulations demonstrated that within a delicate 'window' of gap junctional conductance small clusters of spontaneously active cells are able to induce triggered activity in quiescent mature myocytes but also in a two-dimensional sheet of ventricular cells.

CONCLUSION

Spontaneously active immature cardiomyocytes are able to trigger mature cardiomyocytes depending on their level of electrical coupling and the amount of coupled immature myocytes.

Inhibition of cardiomyocyte automaticity by electrotonic application of inward rectifier current from Kir2.1 expressing cells

A biological pacemaker might be created by generation of a cellular construct consisting of cardiac cells that display spontaneous membrane depolarization, and that are electrotonically coupled to surrounding myocardial cells by means of gap junctions. Depending on the frequency of the spontaneously beating cells, frequency regulation might be required. We hypothesized that application of Kir2.1 expressing non-cardiac cells, which provide I (K1) to spontaneously active neonatal cardiomyocytes (NCMs) by electrotonic coupling in such a cellular construct, would generate an opportunity for pacemaker frequency control. Non-cardiac Kir2.1 expressing cells were co-cultured with spontaneously active rat NCMs. Electrotonic coupling between the two cell types resulted in hyperpolarization of the cardiomyocyte membrane potential and silencing of spontaneous activity. Either blocking of gap-junctional communication by halothane or inhibition of I (K1) by BaCl(2) restored the original membrane potential and spontaneous activity of the NCMs. Our results demonstrate the power of electrotonic coupling for the application of specific ion currents into an engineered cellular construct such as a biological pacemaker.

Xenopus connexins: how frogs bridge the gap

Animal species use specialized cell-to-cell channels, called gap junctions, to allow for a direct exchange of ions and small metabolites between their cells' cytoplasm. In invertebrates, gap junctions are formed by innexins, while vertebrates use connexin (Cx) proteins as gap-junction-building blocks. Recently, innexin homologs have been found in vertebrates and named pannexins. From progress in the different genome projects, it has become evident that every class of vertebrates uses their own unique set of Cxs to build their gap junctions. Here, we review all known Xenopus Cxs with respect to their expression, regulation, and function. We compare Xenopus Cxs with those of zebrafish and mouse, and provide evidence for the existence of several additional, non-identified, amphibian Cxs. Finally, we identify two new Xenopus pannexins by screening EST libraries.