Mouse models of long QT syndrome

Mouse ERG K(+) channel clones reveal differences in protein trafficking and function

Background

The mouse ether‐a‐go‐go‐related gene 1a (mERG1a, mKCNH2) encodes mERG K⁺ channels in mouse cardiomyocytes. The mERG channels and their human analogue, hERG channels, conduct I_Kr. Mutations in hERG channels reduce I_Kr to cause congenital long‐QT syndrome type 2, mostly by decreasing surface membrane expression of trafficking‐deficient channels. Three cDNA sequences were originally reported for mERG channels that differ by 1 to 4 amino acid residues (mERG‐London, mERG‐Waterston, and mERG‐Nie). We characterized these mERG channels to test the postulation that they would differ in their protein trafficking and biophysical function, based on previous findings in long‐QT syndrome type 2.

Methods and Results

The 3 mERG and hERG channels were expressed in HEK293 cells and neonatal mouse cardiomyocytes and were studied using Western blot and whole‐cell patch clamp. We then compared our findings with the recent sequencing results in the Welcome Trust Sanger Institute Mouse Genomes Project (WTSIMGP).

Conclusions

First, the mERG‐London channel with amino acid substitutions in regions of highly ordered structure is trafficking deficient and undergoes temperature‐dependent and pharmacological correction of its trafficking deficiency. Second, the voltage dependence of channel gating would be different for the 3 mERG channels. Third, compared with the WTSIMGP data set, the mERG‐Nie clone is likely to represent the wild‐type mouse sequence and physiology. Fourth, the WTSIMGP analysis suggests that substrain‐specific sequence differences in mERG are a common finding in mice. These findings with mERG channels support previous findings with hERG channel structure–function analyses in long‐QT syndrome type 2, in which sequence changes in regions of highly ordered structure are likely to result in abnormal protein trafficking.

Utility of Human Stem Cells for Drug Discovery

The pharmaceutical industry continues to struggle to deliver novel and innovative medicines to the market. One of the major challenges in deriving new therapeutics is to more accurately predict the safety and efficacy of the candidate molecule. The current paradigm of drug discovery has several limitations but perhaps the most conspicuous deficiency is the lack of human-based experimental models. The advent of human embryonic stem cells followed by the discovery of induced pluripotent stem (iPS) cells offers unprecedented opportunities for integrating human cellular assays in drug discovery and development. Human iPS cell lines of many diseases have been obtained and iPSC-derived disease affected cells have been utilised for proof-of-concept drug screens to assess efficacy or potential toxicology. The incorporation of iPSC technology thus provides an invaluable opportunity to reduce drug attrition during the process of drug development.

Misinterpretation of the mouse ECG: 'musing the waves of Mus musculus'

The ECG is a primary diagnostic tool in patients suffering from heart disease, underscoring the importance of understanding factors contributing to normal and abnormal electrical patterns. Over the past few decades, transgenic mouse models have been increasingly used to study pathophysiological mechanisms of human heart diseases. In order to allow extrapolation of insights gained from murine models to the human condition, knowledge of the similarities and differences between the mouse and human ECG is of crucial importance. In this review, we briefly discuss the physiological mechanisms underlying differences between the baseline ECG of humans and mice, and provide a framework for understanding how these inherent differences are relevant to the interpretation of the mouse ECG during pathology and to the translation of the results from the mouse to man.

Sex differences in the mechanisms underlying long QT syndrome

Sexual dimorphism is a well-established phenomenon, but its degree varies tremendously among species. Since the early days of Einthoven's development of the three-lead galvanometer ECG, we have known there are marked differences in QT intervals of men and women. It required over a century to appreciate the profound implications of sex-based electrophysiological differences in QT interval on the panoply of sex differences with respect to arrhythmia risk, drug sensitivity, and treatment modalities. Little is known about the fundamental mechanism responsible for sex differences in electrical substrate of the human heart, in large part due to the lack of tissue availability. Animal models are an important research tool, but species differences in the sexual dimorphism of the QT interval, the ionic currents underlying the cardiac repolarization, and effects of sex steroids make it difficult to interpolate animal to human sex differences. In addition, in some species, different strains of the same animal model yield conflicting data. Each model has its strengths, such as ease of genetic manipulation in mice or size in dogs. However, many animals do not reproduce the sexual dimorphism of QT seen in humans. To match sex linked prolongation of QT interval and arrhythmogenic phenotype, the current data suggest that the rabbit may be best suited to provide insight into sex differences in humans. In the future, emerging technologies such as induced pluripotent stem cell derived cardiac myocyte systems may offer the opportunity to study sex differences in a controlled hormonal situation in the context of a sex specific human model system.

Speckle-tracking echocardiography elucidates the effect of pacing site on left ventricular synchronization in the normal and infarcted rat myocardium

Background

Right ventricular (RV) pacing generates regional disparities in electrical activation and mechanical function (ventricular dyssynchrony). In contrast, left ventricular (LV) or biventricular (BIV) pacing can improve cardiac efficiency in the setting of ventricular dyssynchrony, constituting the rationale for cardiac resynchronization therapy (CRT). Animal models of ventricular dyssynchrony and CRT currently relay on large mammals which are expensive and not readily available to most researchers. We developed a methodology for double-site epicardial pacing in conscious rats. Here, following post-operative recovery, we compared the effects of various pacing modes on LV dyssynchrony in normal rats and in rats with ischemic cardiomyopathy.

Methods

Two bipolar electrodes were implanted in rats as follows: Group A (n = 6) right atrial (RA) and RV sites; Group B (n = 7) RV and LV sites; Group C (n = 8) as in group B in combination with left coronary artery ligation. Electrodes were exteriorized through the back. Following post-operative recovery, two-dimensional transthoracic echocardiography was performed during pacing through the different electrodes. Segmental systolic circumferential strain (Ecc) was used to evaluate LV dyssynchrony.

Results

In normal rats, RV pacing induced marked LV dyssynchrony compared to RA pacing or sinus rhythm, as measured by the standard deviation (SD) of segmental time to peak Ecc, SD of peak Ecc, and the average delay between opposing ventricular segments. LV pacing and, to a greater extend BIV pacing diminished the LV dyssynchrony compared to RV pacing. In rats with extensive MI, the effects of LV and BIV pacing were markedly attenuated, and the response of individual animals was variable.

Conclusions

Rodent cardiac pacing mimics important features seen in humans. This model may be developed as a simple new tool to study the pathophysiology of ventricular dyssynchrony and CRT.

Organs-on-a-chip: a new tool for drug discovery

INTRODUCTION

The development of emerging in vitro tissue culture platforms can be useful for predicting human response to new compounds, which has been traditionally challenging in the field of drug discovery. Recently, several in vitro tissue-like microsystems, also known as 'organs-on-a-chip', have emerged to provide new tools for better evaluating the effects of various chemicals on human tissue.

AREAS COVERED

The aim of this article is to provide an overview of the organs-on-a-chip systems that have been recently developed. First, the authors introduce single-organ platforms, focusing on the most studied organs such as liver, heart, blood vessels and lung. Later, the authors briefly describe tumor-on-a-chip platforms and highlight their application for testing anti-cancer drugs. Finally, the article reports a few examples of other organs integrated in microfluidic chips along with preliminary multiple-organs-on-a-chip examples. The article also highlights key fabrication points as well as the main application areas of these devices.

EXPERT OPINION

This field is still at an early stage and major challenges need to be addressed prior to the embracement of these technologies by the pharmaceutical industry. To produce predictive drug screening platforms, several organs have to be integrated into a single microfluidic system representative of a humanoid. The routine production of metabolic biomarkers of the organ constructs, as well as their physical environment, have to be monitored prior to and during the delivery of compounds of interest to be able to translate the findings into useful discoveries.

Integrating multi-scale data to create a virtual physiological mouse heart

While the virtual physiological human (VPH) project has made great advances in human modelling, many of the tools and insights developed as part of this initiative are also applicable for facilitating mechanistic understanding of the physiology of a range of other species. This process, in turn, has the potential to provide human relevant insights via a different scientific path. Specifically, the increasing use of mice in experimental research, not yet fully complemented by a similar increase in computational modelling, is currently missing an important opportunity for using and interpreting this growing body of experimental data to improve our understanding of cardiac function. This overview describes our work to address this issue by creating a virtual physiological mouse model of the heart. We describe the similarities between human- and mouse-focused modelling, including the reuse of VPH tools, and the development of methods for investigating parameter sensitivity that are applicable across species. We show how previous results using this approach have already provided important biological insights, and how these can also be used to advance VPH heart models. Finally, we show an example application of this approach to test competing multi-scale hypotheses by investigating variations in length-dependent properties of cardiac muscle.

Injectable 3-D fabrication of medical electronics at the target biological tissues

Conventional transplantable biomedical devices generally request sophisticated surgery which however often causes big trauma and serious pain to the patients. Here, we show an alternative way of directly making three-dimensional (3-D) medical electronics inside the biological body through sequential injections of biocompatible packaging material and liquid metal ink. As the most typical electronics, a variety of medical electrodes with different embedded structures were demonstrated to be easily formed at the target tissues. Conceptual in vitro experiments provide strong evidences for the excellent performances of the injectable electrodes. Further in vivo animal experiments disclosed that the formed electrode could serve as both highly efficient ECG (Electrocardiograph) electrode and stimulator electrode. These findings clarified the unique features and practicability of the liquid metal based injectable 3-D fabrication of medical electronics. The present strategy opens the way for directly manufacturing electrophysiological sensors or therapeutic devices in situ via a truly minimally invasive approach.

Increased persistent sodium current due to decreased PI3K signaling contributes to QT prolongation in the diabetic heart

Diabetes is an independent risk factor for sudden cardiac death and ventricular arrhythmia complications of acute coronary syndrome. Prolongation of the QT interval on the electrocardiogram is also a risk factor for arrhythmias and sudden death, and the increased prevalence of QT prolongation is an independent risk factor for cardiovascular death in diabetic patients. The pathophysiological mechanisms responsible for this lethal complication are poorly understood. Diabetes is associated with a reduction in phosphoinositide 3-kinase (PI3K) signaling, which regulates the action potential duration (APD) of individual myocytes and thus the QT interval by altering multiple ion currents, including the persistent sodium current INaP. Here, we report a mechanism for diabetes-induced QT prolongation that involves an increase in INaP caused by defective PI3K signaling. Cardiac myocytes of mice with type 1 or type 2 diabetes exhibited an increase in APD that was reversed by expression of constitutively active PI3K or intracellular infusion of phosphatidylinositol 3,4,5-trisphosphate (PIP3), the second messenger produced by PI3K. The diabetic myocytes also showed an increase in INaP that was reversed by activated PI3K or PIP3. The increases in APD and INaP in myocytes translated into QT interval prolongation for both types of diabetic mice. The long QT interval of type 1 diabetic hearts was shortened by insulin treatment ex vivo, and this effect was blocked by a PI3K inhibitor. Treatment of both types of diabetic mouse hearts with an INaP blocker also shortened the QT interval. These results indicate that downregulation of cardiac PI3K signaling in diabetes prolongs the QT interval at least in part by causing an increase in INaP. This mechanism may explain why the diabetic population has an increased risk of life-threatening arrhythmias.

Modeling type 3 long QT syndrome with cardiomyocytes derived from patient-specific induced pluripotent stem cells

BACKGROUND

Type 3 long QT syndrome (LQT3) is the third most common form of LQT syndrome and is characterized by QT-interval prolongation resulting from a gain-of-function mutation in SCN5A. We aimed to establish a patient-specific human induced pluripotent stem cell (hiPSC) model of LQT3, which could be used for future drug testing and development of novel treatments for this inherited disorder.

METHODS AND RESULTS

Dermal fibroblasts obtained from a patient with LQT3 harboring a SCN5A mutation (c.5287G>A; p.V1763M) were reprogrammed to hiPSCs via repeated transfection of mRNA encoding OCT-4, SOX-2, KLF-4, C-MYC and LIN-28. hiPSC-derived cardiomyocytes (hiPSC-CMs) were obtained via cardiac differentiation. hiPSC-CMs derived from the patient's healthy sister were used as a control. Compared to the control, patient hiPSC-CMs exhibited dominant mutant SCN5A allele gene expression, significantly prolonged action potential duration or APD (paced CMs of control vs. patient: 226.50 ± 17.89 ms vs. 536.59 ± 37.1 ms; mean ± SEM, p < 0.005), an increased tetrodotoxin (TTX)-sensitive late or persistent Na(+) current (control vs. patient: 0.65 ± 0.11 vs. 3.16 ± 0.27 pA/pF; n = 9, p < 0.01), a positive shift of steady state inactivation and a faster recovery from inactivation. Mexiletine, a NaV1.5 blocker, reversed the elevated late Na(+) current and prolonged APD in LQT3 hiPSC-CMs.

CONCLUSIONS

We demonstrate that hiPSC-CMs derived from a LQT3 patient recapitulate the biophysical abnormalities that define LQT3. The clinical significance of such an in vitro model is in the development of novel therapeutic strategies and a more personalized approach in testing drugs on patients with LQT3.

Modeling the effects of the circadian clock on cardiac electrophysiology

An internal circadian clock regulates the electrical activity of cardiac myocytes controlling the expression of potassium channel interacting protein-2 (KChIP2), which is a key regulator of cardiac electrical activity. Here, we examine how the circadian rhythm of KChIP2 expression affects the dynamics of human and murine ventricular action potentials (APs), as well as the intervals in the equivalent electrocardiograms (ECGs) reflecting the duration of depolarization and repolarization phases of the cardiac ventricular APs (QRS and QT intervals), with mathematical modeling. We show how the internal circadian clock can control the shape of APs and, in particular, predict AP, QRS, and QT interval prolongation following KChIP2 downregulation, as well as shortening of AP, QRS, and QT interval duration following KChIP2 upregulation. Based on the circadian expression of KChIP2, we can accurately predict the circadian rhythm in cardiac electrical activity and suggest the transient outward potassium currents as the key current for circadian rhythmicity. Our modeling work predicts a smaller effect of KChIP2 on AP and QT interval dynamics in humans. Taken together, these results support the role of KChIP2 as the key regulator of circadian rhythms in the electrical activity of the heart; we provide computational models that can be used to explore circadian rhythms in cardiac electrophysiology and susceptibility to arrhythmia.

The role of ion channelopathies in sudden cardiac death: implications for clinical practice

Sudden cardiac death (SCD) following ventricular tachyarrhythmias constitutes an important clinical cause of mortality; 4% of cases may involve ion channel-mediated cellular excitation in structurally normal hearts. Alterations in such processes could disturb action potential conduction, depolarization/ repolarization gradients, or Ca(2+) homeostasis with potential arrhythmogenic consequences. Although SCD may be the first presentation of arrhythmic syndromes, patients may present to the general physician with symptoms of palpitations or hemodynamic compromise, including dizziness, seizure, or syncope, particularly following exertion. In all inherited cardiac death syndromes, first-degree relatives should be referred to a cardiologist and should undergo testing appropriate for the condition. While management of patients at risk of SCD largely centers on risk stratification and, if necessary, insertion of an implantable cardioverter-defibrillator, there are a number of other, pharmacological, treatments being developed. Furthermore, as the genetic basis of these diseases becomes established, genetic testing will form an increasingly important part of diagnosis, and gene-specific therapy is an area under investigation. This article bridges the gap between molecular medicine and clinical practice by reviewing recent developments in the pathophysiological understanding of SCD, and their implications for the management of patients with these complex diseases.

Impaired contractile function due to decreased cardiac myosin binding protein C content in the sarcomere

Mutations in cardiac myosin binding protein C (MyBP-C) are a common cause of familial hypertrophic cardiomyopathy (FHC). The majority of MyBP-C mutations are expected to reduce MyBP-C expression; however, the consequences of MyBP-C deficiency on the regulation of myofilament function, Ca²⁺ homeostasis, and in vivo cardiac function are unknown. To elucidate the effects of decreased MyBP-C expression on cardiac function, we employed MyBP-C heterozygous null (MyBP-C+/-) mice presenting decreases in MyBP-C expression (32%) similar to those of FHC patients carrying MyBP-C mutations. The levels of MyBP-C phosphorylation were reduced 53% in MyBP-C+/- hearts compared with wild-type hearts. Skinned myocardium isolated from MyBP-C+/- hearts displayed decreased cross-bridge stiffness at half-maximal Ca²⁺ activations, increased steady-state force generation, and accelerated rates of cross-bridge recruitment at low Ca²⁺ activations (<15% and <25% of maximum, respectively). Protein kinase A treatment abolished basal differences in rates of cross-bridge recruitment between MyBP-C+/- and wild-type myocardium. Intact ventricular myocytes from MyBP-C+/- hearts displayed abnormal sarcomere shortening but unchanged Ca²⁺ transient kinetics. Despite a lack of left ventricular hypertrophy, MyBP-C+/- hearts exhibited elevated end-diastolic pressure and decreased peak rate of LV pressure rise, which was normalized following dobutamine infusion. Furthermore, electrocardiogram recordings in conscious MyBP-C+/- mice revealed prolonged QRS and QT intervals, which are known risk factors for cardiac arrhythmia. Collectively, our data show that reduced MyBP-C expression and phosphorylation in the sarcomere result in myofilament dysfunction, contributing to contractile dysfunction that precedes compensatory adaptations in Ca²⁺ handling, and chamber remodeling. Perturbations in mechanical and electrical activity in MyBP-C+/- mice could increase their susceptibility to cardiac dysfunction and arrhythmia.

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.

Induced pluripotent stem cell-derived cardiomyocytes: a versatile tool for arrhythmia research

Induced pluripotent stem cells offer the possibility to generate patient-specific stem cell lines from individuals affected by inherited disorders. Cardiomyocytes differentiated from such patient-specific induced pluripotent stem cells lines have been used to study the pathophysiology of arrhythmogenic heart diseases, such as the long-QT syndrome or catecholaminergic polymorphic ventricular tachycardia. Testing for unwanted drug side effects or tailoring medical treatment to the specific needs of individual patients with arrhythmogenic disorders may become future applications of this emerging technology.

Allele-specific RNA interference rescues the long-QT syndrome phenotype in human-induced pluripotency stem cell cardiomyocytes

Aims

Long-QT syndromes (LQTS) are mostly autosomal-dominant congenital disorders associated with a 1:1000 mutation frequency, cardiac arrest, and sudden death. We sought to use cardiomyocytes derived from human-induced pluripotency stem cells (hiPSCs) as an in vitro model to develop and evaluate gene-based therapeutics for the treatment of LQTS.

Methods and results

We produced LQTS-type 2 (LQT2) hiPSC cardiomyocytes carrying a KCNH2 c.G1681A mutation in a I_Kr ion-channel pore, which caused impaired glycosylation and channel transport to cell surface. Allele-specific RNA interference (RNAi) directed towards the mutated KCNH2 mRNA caused knockdown, while leaving the wild-type mRNA unaffected. Electrophysiological analysis of patient-derived LQT2 hiPSC cardiomyocytes treated with mutation-specific siRNAs showed normalized action potential durations (APDs) and K⁺ currents with the concurrent rescue of spontaneous and drug-induced arrhythmias (presented as early-afterdepolarizations).

Conclusions

These findings provide in vitro evidence that allele-specific RNAi can rescue diseased phenotype in LQTS cardiomyocytes. This is a potentially novel route for the treatment of many autosomal-dominant-negative disorders, including those of the heart.

Interpreting genetic effects through models of cardiac electromechanics

Multiscale models of cardiac electromechanics are being increasingly focused on understanding how genetic variation and environment underpin multiple disease states. In this paper we review the current state of the art in both the development of specific models and the physiological insights they have produced. This growing research body includes the development of models for capturing the effects of changes in function in both single and multiple proteins in both specific expression systems and in vivo contexts. Finally, the potential for using this approach for ultimately predicting phenotypes from genetic sequence information is discussed.

Electrocardiograms corresponding to the development of myocardial infarction in anesthetized WHHLMI rabbits (Oryctolagus cuniculus), an animal model for familial hypercholesterolemia

The aim of this study was to determine whether features indicative of myocardial ischemia occur in the electrocardiograms (ECG) in myocardial infarction-prone Watanabe heritable hyperlipidemic (WHHLMI) rabbits, an animal model for human familial hypercholesterolemia. ECG were recorded in 110 anesthetized WHHLMI rabbits (age, 10 to 39 mo) by using unipolar and bipolar limb leads with or without chest leads. We noted the following electrocardiographic changes: T wave inversion (37.4%), ST segment depression (31.8%), deep Q wave (16.3%), reduced R wave amplitude (7.3%), ST segment elevation (2.7%), and high T wave (1.8%). These ECG changes resembled those in human patients with coronary heart disease. Histopathologic examination revealed that the left ventricular wall showed acute myocardial lesions, including loss of cross-striations, vacuolar degeneration, coagulation necrosis of cardiac myocytes, and edema between myofibrils, in addition to chronic myocardial lesions such as myocardial fibrosis. The coronary arteries that caused these ECG changes were severely stenosed due to atherosclerotic lesions. Ischemic ECG changes corresponded to the locations of the myocardial lesions. Normal ECG waveforms were similar between WHHLMI rabbits and humans, in contrast to the large differences between rabbits and mice or rats. In conclusion, ischemic ECG changes in WHHLMI rabbits reflect the location of myocardial lesions, making this model useful for studying coronary heart disease.

Microsystems for biomimetic stimulation of cardiac cells

The heart is a complex integrated system that leverages mechanoelectrical signals to synchronize cardiomyocyte contraction and push blood throughout the body. The correct magnitude, timing, and distribution of these signals is critical for proper functioning of the heart; aberrant signals can lead to acute incidents, long-term pathologies, and even death. Due to the heart's limited regenerative capacity and the wide variety of pathologies, heart disease is often studied in vitro. However, it is difficult to accurately replicate the cardiac environment outside of the body. Studying the biophysiology of the heart in vitro typically consists of studying single cells in a tightly controlled static environment or whole tissues in a complex dynamic environment. Micro-electromechanical systems (MEMS) allow us to bridge these two extremes by providing increasing complexity for cell culture without having to use a whole tissue. Here, we carefully describe the electromechanical environment of the heart and discuss MEMS specifically designed to replicate these stimulation modes. Strengths, limitations and future directions of various designs are discussed for a variety of applications.

Cardiac electrophysiology in mice: a matter of size

Over the last decade, mouse models have become a popular instrument for studying cardiac arrhythmias. This review assesses in which respects a mouse heart is a miniature human heart, a suitable model for studying mechanisms of cardiac arrhythmias in humans and in which respects human and murine hearts differ. Section I considers the issue of scaling of mammalian cardiac (electro) physiology to body mass. Then, we summarize differences between mice and humans in cardiac activation (section II) and the currents underlying the action potential in the murine working myocardium (section III). Changes in cardiac electrophysiology in mouse models of heart disease are briefly outlined in section IV, while section V discusses technical considerations pertaining to recording cardiac electrical activity in mice. Finally, section VI offers general considerations on the influence of cardiac size on the mechanisms of tachy-arrhythmias.

Sudden cardiac death and inherited channelopathy: the basic electrophysiology of the myocyte and myocardium in ion channel disease

Mutations involving cardiac ion channels result in abnormal action potential formation or propagation, leading to cardiac arrhythmias. Despite the large impact on society of sudden cardiac death resulting from such arrhythmias, understanding of the underlying cellular mechanism is poor and clinical risk stratification and treatment consequently limited. Basic research using molecular techniques, as well as animal models, has proved extremely useful in improving our knowledge of inherited arrhythmogenic syndromes. This offers the practitioner tools to accurately diagnose rare disorders and provides novel markers for risk assessment and a basis for new strategies of treatment.

Suppression of phosphoinositide 3-kinase signaling and alteration of multiple ion currents in drug-induced long QT syndrome

Many drugs, including some commonly used medications, can cause abnormal heart rhythms and sudden death, as manifest by a prolonged QT interval in the electrocardiogram. Cardiac arrhythmias caused by drug-induced long QT syndrome are thought to result mainly from reductions in the delayed rectifier potassium ion (K(+)) current I(Kr). Here, we report a mechanism for drug-induced QT prolongation that involves changes in multiple ion currents caused by a decrease in phosphoinositide 3-kinase (PI3K) signaling. Treatment of canine cardiac myocytes with inhibitors of tyrosine kinases or PI3Ks caused an increase in action potential duration that was reversed by intracellular infusion of phosphatidylinositol 3,4,5-trisphosphate. The inhibitors decreased the delayed rectifier K(+) currents I(Kr) and I(Ks), the L-type calcium ion (Ca(2+)) current I(Ca,L), and the peak sodium ion (Na(+)) current I(Na) and increased the persistent Na(+) current I(NaP). Computer modeling of the canine ventricular action potential showed that the drug-induced change in any one current accounted for less than 50% of the increase in action potential duration. Mouse hearts lacking the PI3K p110α catalytic subunit exhibited a prolonged action potential and QT interval that were at least partly a result of an increase in I(NaP). These results indicate that down-regulation of PI3K signaling directly or indirectly via tyrosine kinase inhibition prolongs the QT interval by affecting multiple ion channels. This mechanism may explain why some tyrosine kinase inhibitors in clinical use are associated with increased risk of life-threatening arrhythmias.

On the emerging role of rabbit as human disease model and the instrumental role of novel transgenic tools

The laboratory rabbit (Oryctolagus cuniculus) is widely used as a model for human diseases, because of its size, which permits non-lethal monitoring of physiological changes and similar disease characteristics. Novel transgenic tools such as, the zinc finger nuclease method and the sleeping beauty transposon mediated or BAC transgenesis were recently adapted to the laboratory rabbit and opened new opportunities in precise tissue and developmental stage specific gene expression/silencing, coupled with increased transgenic efficiencies. Many facets of human development and diseases cannot be investigated in rodents. This is especially true for early prenatal development, its long-lasting effects on health and complex disorders, and some economically important diseases such as atherosclerosis or cardiovascular diseases. The first transgenic rabbits models of arrhythmogenesis mimic human cardiac diseases much better than transgenic mice and hereby underline the importance of non-mouse models. Another emerging field is epigenetic reprogramming and pathogenic mechanisms in diabetic pregnancy, where rabbit models are indispensable. Beyond that rabbit is used for decades as major source of polyclonal antibodies and recently in monoclonal antibody production. Alteration of its genome to increase the efficiency and value of the antibodies by humanization of the immunoglobulin genes, or by increasing the expression of a special receptor (Fc receptor) that augments humoral immune response is a current demand.

Proarrhythmia in a non-failing murine model of cardiac-specific Na+/Ca 2+ exchanger overexpression: whole heart and cellular mechanisms

The cardiac Na(+)/Ca(2+) exchanger (NCX) generates an inward electrical current during SR-Ca(2+) release, thus possibly promoting afterdepolarizations of the action potential (AP). We used transgenic mice 12.5 weeks or younger with cardiomyocyte-directed overexpression of NCX (NCX-Tg) to study the proarrhythmic potential and mechanisms of enhanced NCX activity. NCX-Tg exhibited normal echocardiographic left ventricular function and heart/body weight ratio, while the QT interval was prolonged in surface ECG recordings. Langendorff-perfused NCX-Tg, but not wild-type (WT) hearts, developed ventricular tachycardia. APs and ionic currents were measured in isolated cardiomyocytes. Cell capacitance was unaltered between groups. APs were prolonged in NCX-Tg versus WT myocytes along with voltage-activated K(+) currents (K(v)) not being reduced but even increased in amplitude. During abrupt changes in pacing cycle length, early afterdepolarizations (EADs) were frequently recorded in NCX-Tg but not in WT myocytes. Next to EADs, delayed afterdepolarizations (DAD) triggering spontaneous APs (sAPs) occurred in NCX-Tg but not in WT myocytes. To test whether sAPs were associated with spontaneous Ca(2+) release (sCR), Ca(2+) transients were recorded. Despite the absence of sAPs in WT, sCR was observed in myocytes of both genotypes suggesting a facilitated translation of sCR into DADs in NCX-Tg. Moreover, sCR was more frequent in NCX-Tg as compared to WT. Myocardial protein levels of Ca(2+)-handling proteins were not different between groups except the ryanodine receptor (RyR), which was increased in NCX-Tg versus WT. We conclude that NCX overexpression is proarrhythmic in a non-failing environment even in the absence of reduced K(V). The underlying mechanisms are: (1) occurrence of EADs due to delayed repolarization; (2) facilitated translation from sCR into DADs; (3) proneness to sCR possibly caused by altered Ca(2+) handling and/or increased RyR expression.

The Popeye domain containing 2 (popdc2) gene in zebrafish is required for heart and skeletal muscle development

The Popeye domain containing (Popdc) genes encode a family of transmembrane proteins with an evolutionary conserved Popeye domain. These genes are abundantly expressed in striated muscle tissue, however their function is not well understood. In this study we have investigated the role of the popdc2 gene in zebrafish. Popdc2 transcripts were detected in the embryonic myocardium and transiently in the craniofacial and tail musculature. Morpholino oligonucleotide-mediated knockdown of popdc2 resulted in aberrant development of skeletal muscle and heart. Muscle segments in the trunk were irregularly shaped and craniofacial muscles were severely reduced or even missing. In the heart, pericardial edema was prevalent in the morphants and heart chambers were elongated and looping was abnormal. These pathologies in muscle and heart were alleviated after reducing the morpholino concentration. However the heart still was abnormal displaying cardiac arrhythmia at later stages of development. Optical recordings of cardiac contractility revealed irregular ventricular contractions with a 2:1, or 3:1 atrial/ventricular conduction ratio, which caused a significant reduction in heart frequency. Recordings of calcium transients with high spatiotemporal resolution using a transgenic calcium indicator line (Tg(cmlc2:gCaMP)(s878)) and SPIM microscopy confirmed the presence of a severe arrhythmia phenotype. Our results identify popdc2 as a gene important for striated muscle differentiation and cardiac morphogenesis. In addition it is required for the development of the cardiac conduction system.

Mice with cardiac overexpression of peroxisome proliferator-activated receptor γ have impaired repolarization and spontaneous fatal ventricular arrhythmias

BACKGROUND

Diabetes mellitus and obesity, which confer an increased risk of sudden cardiac death, are associated with cardiomyocyte lipid accumulation and altered cardiac electric properties, manifested by prolongation of the QRS duration and QT interval. It is difficult to distinguish the contribution of cardiomyocyte lipid accumulation from the contribution of global metabolic defects to the increased incidence of sudden death and electric abnormalities.

METHODS AND RESULTS

In order to study the effects of metabolic abnormalities on arrhythmias without the complex systemic effects of diabetes mellitus and obesity, we studied transgenic mice with cardiac-specific overexpression of peroxisome proliferator-activated receptor γ 1 (PPARγ1) via the cardiac α-myosin heavy-chain promoter. The PPARγ transgenic mice develop abnormal accumulation of intracellular lipids and die as young adults before any significant reduction in systolic function. Using implantable ECG telemeters, we found that these mice have prolongation of the QRS and QT intervals and spontaneous ventricular arrhythmias, including polymorphic ventricular tachycardia and ventricular fibrillation. Isolated cardiomyocytes demonstrated prolonged action potential duration caused by reduced expression and function of the potassium channels responsible for repolarization. Short-term exposure to pioglitazone, a PPARγ agonist, had no effect on mortality or rhythm in WT mice but further exacerbated the arrhythmic phenotype and increased the mortality in the PPARγ transgenic mice.

CONCLUSIONS

Our findings support an important link between PPARγ activation, cardiomyocyte lipid accumulation, ion channel remodeling, and increased cardiac mortality.

Triple threat: the Na+/Ca2+ exchanger in the pathophysiology of cardiac arrhythmia, ischemia and heart failure

The Na(+)/Ca(2+) exchanger (NCX) is the main Ca(2+) extrusion mechanism of the cardiac myocyte and thus is crucial for maintaining Ca(2+) homeostasis. It is involved in the regulation of several parameters of cardiac excitation contraction coupling, such as cytosolic Ca(2+) concentration, repolarization and contractility. Increased NCX activity has been identified as a mechanism promoting heart failure, cardiac ischemia and arrhythmia. Transgenic mice as well as pharmacological interventions have been used to support the idea of using NCX inhibition as a future pharmacological strategy to treat cardiovascular disease.

Deciphering Arrhythmia Mechanisms - Tools of the Trade

Pathophysiological remodeling of cardiac function occurs at multiple levels, spanning the spectrum from molecular and sub-cellular changes to those occurring at the organ-system levels. Of key importance to arrhythmias are changes in electrophysiological and calcium handling properties at the tissue level. In this review, we discuss how high-resolution optical action potential and calcium transient imaging has advanced our understanding of basic arrhythmia mechanisms associated with multiple cardiovascular disorders, including the long QT syndrome, heart failure, and ischemia-reperfusion injury. We focus on the role of repolarization gradients (section 1) and calcium mediated triggers (section 2) in the initiation and maintenance of complex arrhythmias in these settings.

Upregulation of ventricular potassium channels by chronic tamoxifen treatment

AIMS

Tamoxifen is a selective oestrogen receptor modulator widely used in the prevention and treatment of breast cancer. Women receiving long-term tamoxifen therapy do not experience cardiac arrhythmias although acute perfusion of tamoxifen has been shown to inhibit cardiac K(+) currents. This observation suggests that chronic tamoxifen treatment does not negatively modulate cardiac K(+) currents. Therefore, we investigated the chronic effects of tamoxifen on K(+) currents and channels in mouse and guinea pig ventricles.

METHODS AND RESULTS

Female mice and guinea pigs were treated with placebo or tamoxifen pellets for 60 days. Voltage-clamp experiments showed that the density of the Ca²(+)-independent transient outward (I(to)), the ultrarapid delayed rectifier (I(Kur)), the steady-state (I(ss)), and the inward rectifier (I(K1)) K(+) currents were increased in tamoxifen-treated mice ventricle. Western blot analysis revealed that protein expression of the underlying K(+) channels Kv4.3 (I(to)), Kv1.5 (I(Kur)), Kv2.1 (I(ss)), and Kir2.1 (I(K1)) were significantly higher in the ventricle of tamoxifen-treated mice. Protein expression of the K(+) channel subunits encoding I(Kr) and I(Ks) (ERG1, KCNQ1, and KCNE1) was also increased in tamoxifen-treated guinea pig ventricle.

CONCLUSION

Conditions with high oestrogen levels are associated with reduced K(+) currents. Thus, conceivably, tamoxifen might prevent the inhibitory effects of oestrogen on K(+) channels by blocking the oestrogen receptors, which would explain the reported increase in K(+) currents. These findings could contribute to explain the absence of cardiac arrhythmia with long-term tamoxifen therapy.

A β(IV)-spectrin/CaMKII signaling complex is essential for membrane excitability in mice

Ion channel function is fundamental to the existence of life. In metazoans, the coordinate activities of voltage-gated Na(+) channels underlie cellular excitability and control neuronal communication, cardiac excitation-contraction coupling, and skeletal muscle function. However, despite decades of research and linkage of Na(+) channel dysfunction with arrhythmia, epilepsy, and myotonia, little progress has been made toward understanding the fundamental processes that regulate this family of proteins. Here, we have identified β(IV)-spectrin as a multifunctional regulatory platform for Na(+) channels in mice. We found that β(IV)-spectrin targeted critical structural and regulatory proteins to excitable membranes in the heart and brain. Animal models harboring mutant β(IV)-spectrin alleles displayed aberrant cellular excitability and whole animal physiology. Moreover, we identified a regulatory mechanism for Na(+) channels, via direct phosphorylation by β(IV)-spectrin-targeted calcium/calmodulin-dependent kinase II (CaMKII). Collectively, our data define an unexpected but indispensable molecular platform that determines membrane excitability in the mouse heart and brain.

Molecular genetics of long QT syndrome

Long QT syndrome (LQTS) is a cardiac disorder associated with sudden death especially in young, seemingly healthy individuals. It is characterised by abnormalities of the heart beat detected as lengthening of the QT interval during cardiac repolarisation. The incidence of LQTS is given as 1 in 2000 but this may be an underestimation as many cases go undiagnosed, due to the rarity of the condition and the wide spectrum of symptoms. Presently 12 genes associated with LQTS have been identified with differing signs and symptoms, depending on the locus involved. The majority of cases have mutations in the KCNQ1 (LQT1), KCNH2 (LQT2) and SCN5A (LQT3) genes. Genetic testing is increasingly used when a clearly affected proband has been identified, to determine the nature of the mutation in that family. Unfortunately tests on probands may be uninformative, especially if the defect does not lie in the set of genes which are routinely tested. Novel mutations in these known LQTS genes and additional candidate genes are still being discovered. The functional implications of these novel mutations need to be assessed before they can be accepted as being responsible for LQTS. Known epigenetic modification affecting KCNQ1 gene expression may also be involved in phenotypic variability of LQTS. Genetic diagnosis of LQTS is thus challenging. However, where a disease associated mutation is identified, molecular diagnosis can be important in guiding therapy, in family testing and in determining the cause of sudden cardiac death. New developments in technology and understanding offer increasing hope to families with this condition.

Zebrafish as a model for long QT syndrome: the evidence and the means of manipulating zebrafish gene expression

Congenital long QT syndrome (LQT) is a group of cardiac disorders associated with the dysfunction of cardiac ion channels. It is characterized by prolongation of the QT-interval, episodes of syncope and even sudden death. Individuals may remain asymptomatic for most of their lives while others present with severe symptoms. This heterogeneity in phenotype makes diagnosis difficult with a greater emphasis on more targeted therapy. As a means of understanding the molecular mechanisms underlying LQT syndrome, evaluating the effect of modifier genes on disease severity as well as to test new therapies, the development of model systems remains an important research tool. Mice have predominantly been the animal model of choice for cardiac arrhythmia research, but there have been varying degrees of success in recapitulating the human symptoms; the mouse cardiac action potential (AP) and surface electrocardiograms exhibit major differences from those of the human heart. Against this background, the zebrafish is an emerging vertebrate disease modelling species that offers advantages in analysing LQT syndrome, not least because its cardiac AP much more closely resembles that of the human. This article highlights the use and potential of this species in LQT syndrome modelling, and as a platform for the in vivo assessment of putative disease-causing mutations in LQT genes, and of therapeutic interventions.

Cardiac Arrhythmia: In vivo screening in the zebrafish to overcome complexity in drug discovery

IMPORTANCE OF THE FIELD: Cardiac arrhythmias remain a major challenge for modern drug discovery. Clinical events are paroxysmal, often rare and may be asymptomatic until a highly morbid complication. Target selection is often based on limited information and though highly specific agents are identified in screening, the final efficacy is often compromised by unanticipated systemic responses, a narrow therapeutic index and substantial toxicities. AREAS COVERED IN THIS REVIEW: Our understanding of complexity of arrhythmogenesis has grown dramatically over the last two decades, and the range of potential disease mechanisms now includes pathways previously thought only tangentially involved in arrhythmia. This review surveys the literature on arrhythmia mechanisms from 1965 to the present day, outlines the complex biology underlying potentially each and every rhythm disturbance, and highlights the problems for rational target identification. The rationale for in vivo screening is described and the utility of the zebrafish for this approach and for complementary work in functional genomics is discussed. Current limitations of the model in this setting and the need for careful validation in new disease areas are also described. WHAT THE READER WILL GAIN: An overview of the complex mechanisms underlying most clinical arrhythmias, and insight into the limits of ion channel conductances as drug targets. An introduction to the zebrafish as a model organism, in particular for cardiovascular biology. Potential approaches to overcoming the hurdles to drug discovery in the face of complex biology including in vivo screening of zebrafish genetic disease models. TAKE HOME MESSAGE: In vivo screening in faithful disease models allows the effects of drugs on integrative physiology and disease biology to be captured during the screening process, in a manner agnostic to potential drug target or targets. This systematic strategy bypasses current gaps in our understanding of disease biology, but emphasizes the importance of the rigor of the disease model.

Absence of the inhibitory G-protein Galphai2 predisposes to ventricular cardiac arrhythmia

BACKGROUND

We explored the role that inhibitory heterotrimeric G-proteins play in ventricular arrhythmia.

METHODS AND RESULTS

Mice with global genetic deletion of Galpha(i2) [Galpha(i2) (-/-)] were studied and found, based on telemetry, to have a prolonged QT interval on surface ECG when awake. In vivo electrophysiology studies revealed that the Galpha(i2) (-/-) mice have a reduced ventricular effective refractory period and a predisposition to ventricular tachycardia when challenged with programmed electrical stimulation. Neither control nor combined global deletion of Galpha(i1) and Galpha(i3) mice showed these abnormalities. There was no evidence for structural heart disease at this time point in the Galpha(i2) (-/-) mice as assessed by cardiac histology and echocardiography. The absence of Galpha(i2) thus leads to a primary electrical abnormality, and we explored the basis for this finding. With patch clamping, single isolated ventricular cells showed that Galpha(i2) (-/-) mice had a prolonged ventricular action potential duration (APD) but steeper action potential shortening as the diastolic interval was reduced in restitution studies. Gene expression studies showed increased expression of L-type Ca(2+) channel subunits, and patch clamping revealed an increase in these currents in Galpha(i2) (-/-) mice. There were no changes in K(+) currents.

CONCLUSIONS

The absence of inhibitory G-protein signaling mediated through Galpha(i2) is a substrate for ventricular arrhythmias.

Anthropomorphizing the mouse cardiac action potential via a novel dynamic clamp method

Interspecies differences can limit the translational value of excitable cells isolated from model organisms. It can be difficult to extrapolate from a drug- or mutation-induced phenotype in mice to human pathophysiology because mouse and human cardiac electrodynamics differ greatly. We present a hybrid computational-experimental technique, the cell-type transforming clamp, which is designed to overcome such differences by using a calculated compensatory current to convert the macroscopic electrical behavior of an isolated cell into that of a different cell type. We demonstrate the technique's utility by evaluating drug arrhythmogenicity in murine cardiomyocytes that are transformed to behave like human myocytes. Whereas we use the cell-type transforming clamp in this work to convert between mouse and human electrodynamics, the technique could be adapted to convert between the action potential morphologies of any two cell types of interest.

Homeostatic control of sensory output in basal vomeronasal neurons: activity-dependent expression of ether-à-go-go-related gene potassium channels

Conspecific chemosensory communication controls a broad range of social and sexual behaviors. In most mammals, social chemosignals are predominantly detected by sensory neurons of a specialized olfactory subsystem, the vomeronasal organ (VNO). The behavioral relevance of social chemosignaling puts high demands on the accuracy and dynamic range of the underlying transduction mechanisms. However, the physiological concepts implemented to ensure faithful transmission of social information remain widely unknown. Here, we show that sensory neurons in the basal layer of the mouse VNO dynamically control their input-output relationship by activity-dependent regulation of K(+) channel gene expression. Using large-scale expression profiling, immunochemistry, and electrophysiology, we provide molecular and functional evidence for a role of ether-à-go-go-related gene (ERG) K(+) channels as key determinants of cellular excitability. Our findings indicate that an increase in ERG channel expression extends the dynamic range of the stimulus-response function in basal vomeronasal sensory neurons. This novel mechanism of homeostatic plasticity in the periphery of the accessory olfactory system is ideally suited to adjust VNO neurons to a target output range in a layer-specific and use-dependent manner.

Re-evaluating the efficacy of beta-adrenergic agonists and antagonists in long QT-3 syndrome through computational modelling

AIMS

Long QT syndrome (LQTS) is a heterogeneous collection of inherited cardiac ion channelopathies characterized by a prolonged electrocardiogram QT interval and increased risk of sudden cardiac death. Beta-adrenergic blockers are the mainstay of treatment for LQTS. While their efficacy has been demonstrated in LQTS patients harbouring potassium channel mutations, studies of beta-blockers in subtype 3 (LQT3), which is caused by sodium channel mutations, have produced ambiguous results. In this modelling study, we explore the effects of beta-adrenergic drugs on the LQT3 phenotype.

METHODS AND RESULTS

In order to investigate the effects of beta-adrenergic activity and to identify sources of ambiguity in earlier studies, we developed a computational model incorporating the effects of beta-agonists and beta-blockers into an LQT3 mutant guinea pig ventricular myocyte model. Beta-activation suppressed two arrhythmogenic phenomena, transmural dispersion of repolarization and early after depolarizations, in a dose-dependent manner. However, the ability of beta-activation to prevent cardiac conduction block was pacing-rate-dependent. Low-dose beta-blockade by propranolol reversed the beneficial effects of beta-activation, while high dose (which has off-target sodium channel effects) decreased arrhythmia susceptibility.

CONCLUSION

These results demonstrate that beta-activation may be protective in LQT3 and help to reconcile seemingly conflicting results from different experimental models. They also highlight the need for well-controlled clinical investigations re-evaluating the use of beta-blockers in LQT3 patients.

Zebrafish genetic models for arrhythmia

Over the last decade the zebrafish has emerged as a major genetic model organism. While stimulated originally by the utility of its transparent embryos for the study of vertebrate organogenesis, the success of the zebrafish was consolidated through multiple genetic screens, sequencing of the fish genome by the Sanger Center, and the advent of extensive genomic resources. In the last few years the potential of the zebrafish for in vivo cell biology, physiology, disease modeling and drug discovery has begun to be realized. This review will highlight work on cardiac electrophysiology, emphasizing the arenas in which the zebrafish complements other in vivo and in vitro models; developmental physiology, large-scale screens, high-throughput disease modeling and drug discovery. Much of this work is at an early stage, and so the focus will be on the general principles, the specific advantages of the zebrafish and on future potential.

Atrial arrhythmogenesis in wild-type and Scn5a+/delta murine hearts modelling LQT3 syndrome

Long QT(3) (LQT3) syndrome is associated with abnormal repolarisation kinetics, prolonged action potential durations (APD) and QT intervals and may lead to life-threatening ventricular arrhythmias. However, there have been few physiological studies of its effects on atrial electrophysiology. Programmed electrical stimulation and burst pacing induced atrial arrhythmic episodes in 16 out of 16 (16/16) wild-type (WT) and 7/16 genetically modified Scn5a+/Δ (KPQ) Langendorff-perfused murine hearts modelling LQT3 (P < 0.001 for both), and in 14/16 WT and 1/16 KPQ hearts (P < 0.001 for both; Fisher’s exact test), respectively. The arrhythmogenic WT hearts had significantly larger positive critical intervals (CI), given by the difference between atrial effective refractory periods (AERPs) and action potential durations at 90% recovery (APD₉₀), compared to KPQ hearts (8.1 and 3.2 ms, respectively, P < 0.001). Flecainide prevented atrial arrhythmias in all arrhythmogenic WT (P < 0.001) and KPQ hearts (P < 0.05). It prolonged the AERP to a larger extent than it did the APD₉₀ in both WT and KPQ groups, giving negative CIs. Quinidine similarly exerted anti-arrhythmic effects, prolonged AERP over corresponding APD₉₀ in both WT and KPQ groups. These findings, thus, demonstrate, for the first time, inhibitory effects of the KPQ mutation on atrial arrhythmogenesis and its modification by flecainide and quinidine. They attribute these findings to differences in the CI between WT and mutant hearts, in the presence or absence of these drugs. Thus, prolongation of APD₉₀ over AERP gave positive CI values and increased atrial arrhythmogenicity whereas lengthening of AERP over APD₉₀ reduced such CI values and produced the opposite effect.

Arrhythmia phenotype in mouse models of human long QT

Enhanced dispersion of repolarization (DR) was proposed as a unifying mechanism, central to arrhythmia genesis in the long QT (LQT) syndrome. In mammalian hearts, K(+) channels are heterogeneously expressed across the ventricles resulting in 'intrinsic' DR that may worsen in long QT. DR was shown to be central to the arrhythmia phenotype of transgenic mice with LQT caused by loss of function of the dominant mouse K(+) currents. Here, we investigated the arrhythmia phenotype of mice with targeted deletions of KCNE1 and KCNH2 genes which encode for minK/IsK and Merg1 (mouse homolog of human ERG) proteins resulting in loss of function of I(Ks) and I(Kr), respectively. Both currents are important human K(+) currents associated with LQT5 and LQT2. Loss of minK, a protein subunit that interacts with KvLQT1, results in a marked reduction of I(Ks) giving rise to the Jervell and Lange-Nielsen syndrome and the reduced KCNH2 gene reduces MERG and I(Kr). Hearts were perfused, stained with di-4-ANEPPS and optically mapped to compare action potential durations (APDs) and arrhythmia phenotype in homozygous minK (minK(-/-)) and heterozygous Merg1 (Merg(+/-)) deletions and littermate control mice. MinK(-/-) mice has similar APDs and no arrhythmias (n = 4). Merg(+/-) mice had prolonged APDs (from 20 +/- 6 to 32 +/- 9 ms at the base, p < 0.01; from 18 +/- 5 to 25 +/- 9 ms at the apex, p < 0.01; n = 8), longer refractory periods (RP) (36 +/- 14 to 63 +/- 27 at the base, p < 0.01 and 34 +/- 5 to 53 +/- 21 ms at the apex, p < 0.03; n = 8), higher DR 10.4 +/- 4.1 vs. 14 +/- 2.3 ms, p < 0.02) and similar conduction velocities (n = 8). Programmed stimulation exposed a higher propensity to VT in Merg(+/-) mice (60% vs. 10%). A comparison of mouse models of LQT based on K(+) channel mutations important to human and mouse repolarization emphasizes DR as a major determinant of arrhythmia vulnerability.

New insights into the atrial electrophysiology of rodents using a novel modality: the miniature-bipolar hook electrode

Studies of atrial electrophysiology (EP) in rodents are challenging, and available data are sparse. Herein, we utilized a novel type of bipolar electrode to evaluate the atrial EP of rodents through small lateral thoracotomy. In anesthetized rats and mice, we attached two bipolar electrodes to the right atrium and a third to the right ventricle. This standard setup enabled high-resolution EP studies. Moreover, a permanent implantation procedure enabled EP studies in conscious freely moving rats. Atrial EP was evaluated in anesthetized rats, anesthetized mice (ICR and C57BL6 strains), and conscious rats. Signal resolution enabled atrial effective refractory period (AERP) measurements and first time evaluation of the failed 1:1 atrial capture, which was unexpectedly longer than the AERP recorded at near normal cycle length by 27.2+/-2.3% in rats (P<0.0001; n=35), 31.7+/-8.3% in ICR mice (P=0.0001; n=13), and 57.7+/-13.7% in C57BL6 mice (P=0.015; n=4). While AERP rate adaptation was noted when 10 S1s at near normal basic cycle lengths were followed by S2 at varying basic cycle length and S3 for AERP evaluation, such rate adaptation was absent using conventional S1S2 protocols. Atrial tachypacing in rats shortened the AERP values on a timescale of hours, but a reverse remodeling phase was noted thereafter. Comparison of left vs. right atrial pacing in rats was also feasible with the current technique, resulting in similar AERP values recorded in the low right atrium. In conclusion, our findings indicate that in vivo rate adaptation of the rodent atria is different than expected based on previous ex vivo recordings. In addition, atrial electrical remodeling of rats shows unique remodeling-reverse remodeling characteristics that are described here for the first time. Further understanding of these properties should help to determine the clinical relevance as well as limitations of atrial arrhythmia models in rodents.

Targeted deletion of kcne2 impairs ventricular repolarization via disruption of I(K,slow1) and I(to,f)

Mutations in human KCNE2, which encodes the MiRP1 potassium channel ancillary subunit, associate with long QT syndrome (LQTS), a defect in ventricular repolarization. The precise cardiac role of MiRP1 remains controversial, in part, because it has marked functional promiscuity in vitro. Here, we disrupted the murine kcne2 gene to define the role of MiRP1 in murine ventricles. kcne2 disruption prolonged ventricular action potential duration (APD), suggestive of reduced repolarization capacity. Accordingly, kcne2 (-/-) ventricles exhibited a 50% reduction in I(K,slow1), generated by Kv1.5--a previously unknown partner for MiRP1. I(to,f), generated by Kv4 alpha subunits, was also diminished, by approximately 25%. Ventricular MiRP1 protein coimmunoprecipitated with native Kv1.5 and Kv4.2 but not Kv1.4 or Kv4.3. Unexpectedly, kcne2 (-/-) ventricular membrane fractions exhibited 50% less mature Kv1.5 protein than wild type, and disruption of Kv1.5 trafficking to the intercalated discs. Consistent with the reduction in ventricular K(+) currents and prolonged ventricular APD, kcne2 deletion lengthened the QT(c) under sevoflurane anesthesia. Thus, targeted disruption of kcne2 has revealed a novel cardiac partner for MiRP1, a novel role for MiRPs in alpha subunit targeting in vivo, and a role for MiRP1 in murine ventricular repolarization with parallels to that proposed for the human heart.

Predicting QT prolongation in humans during early drug development using hERG inhibition and an anaesthetized guinea-pig model

Background and purpose:

Drug-induced prolongation of the QT interval can lead to torsade de pointes, a life-threatening ventricular arrhythmia. Finding appropriate assays from among the plethora of options available to predict reliably this serious adverse effect in humans remains a challenging issue for the discovery and development of drugs. The purpose of the present study was to develop and verify a reliable and relatively simple approach for assessing, during preclinical development, the propensity of drugs to prolong the QT interval in humans.

Experimental approach:

Sixteen marketed drugs from various pharmacological classes with a known incidence—or lack thereof—of QT prolongation in humans were examined in hERG (human ether a-go-go-related gene) patch-clamp assay and an anaesthetized guinea-pig assay for QT prolongation using specific protocols. Drug concentrations in perfusates from hERG assays and plasma samples from guinea-pigs were determined using liquid chromatography-mass spectrometry.

Key results:

Various pharmacological agents that inhibit hERG currents prolong the QT interval in anaesthetized guinea-pigs in a manner similar to that seen in humans and at comparable drug exposures. Several compounds not associated with QT prolongation in humans failed to prolong the QT interval in this model.

Conclusions and implications:

Analysis of hERG inhibitory potency in conjunction with drug exposures and QT interval measurements in anaesthetized guinea-pigs can reliably predict, during preclinical drug development, the risk of human QT prolongation. A strategy is proposed for mitigating the risk of QT prolongation of new chemical entities during early lead optimization.

Comparison of the cardiac electrophysiological effects between doxazosin and bunazosin

In Langendorff-perfused adult rat heart with constant pressure at 80 mmHg, we found doxazosin, an alpha(1) adrenoceptor blocker, at 10 muM prolonged PR interval and induced occasional arrhythmia followed by complete inhibition of the sinus rhythm, whereas bunazosin, another alpha(1)-blocker, at same concentration did not. The results of voltage-clamp study showed that, at the concentration of 10 muM, doxazosin inhibited I (Na), I ( (Ca,L) ), I (to), and Iss without changing I (k1) but bunazosin only inhibited I ( (Ca,L) ) about 30%. Doxazosin also caused markedly negative shift of the I (Na) inactivation curve. In current-clamp study, doxazosin prolonged action potential duration in association with the decreased action potential amplitude and upstroke velocity, whereas bunazosin did not. We hypothesize that doxazosin-induced arrhythmia may result from the heterogeneous or different level of I ( (Ca,L) ) blockade of AV nodal tissue. In conclusion, the present study suggests that bunazosin is safer than doxazosin for long-term treatment in view of electrophysiological effect. However, the underlying mechanism of doxazosin induced nodal arrhythmia is still needed to be determined.