Micro-electrocardiograms to study post-ventricular amputation of zebrafish heart

Protium heptaphyllum essential oil from the fruit as a sedative and anesthetic in Rhamdia quelen: influence in cardiac frequency, biochemical, and oxidative parameters

This study aimed to evaluate the effects of Protium heptaphyllum fruit essential oil (PHEO) on the physiology of silver catfish (Rhamdia quelen) during anesthesia and recovery, through studying echocardiograms, oxidative status, and metabolic parameters. Three experiments were performed: (1) 50 silver catfish juveniles were submitted to anesthesia and recovery tests with 300, 400, 500, 600, and 700 mg L-1 of PHEO. (2) Echocardiogram analysis was performed in anesthetized and non-anesthetized fish. (3) Biochemical parameters were evaluated at 0, 30, 60, and 120 min of recovery after being anesthetized for 3 min with 600 mg L-1 PHEO. Times to sedation and deep anesthesia were reduced with PHEO increasing concentrations. The echocardiogram showed a higher cardiac rate in anesthetized fish. Plasma glucose levels increased in control fish through recovery time, but anesthetized fish showed lower levels than controls at 120 min of recovery. Metabolic parameters such as plasma and hepatic glucose did not show changes considering the recovery time of up to 120 min. Hepatic glycogen, lactate, and triglycerides reduced their levels over recovery times. Fish anesthetized enhanced superoxide dismutase activity and thiobarbituric acid reactive substances levels but decreased reduced glutathione (GSH) levels at 30 min compared to controls. After 60 min, GSH values were significantly higher in anesthetized fish than in controls. These results suggest that PHEO at 600 mg L-1 is an effective anesthetic for the rapid handling of silver catfish, providing stable metabolic parameters and enhanced antioxidant responses during recovery. Echocardiogram analysis confirms the anesthetic effect, supporting PHEO as a viable and efficient option for fish anesthesia in aquaculture. The use of PHEO in aquaculture can enhance fish welfare by reducing stress during handling and transportation, potentially leading to improved growth, health, and survival rates.

Consecutive treatments of methamphetamine promote the development of cardiac pathological symptoms in zebrafish

Chronic methamphetamine use, a widespread drug epidemic, has been associated with cardiac morphological and electrical remodeling, leading to the development of numerous cardiovascular diseases. While methamphetamine has been documented to induce arrhythmia, most results originate from clinical trials from users who experienced different durations of methamphetamine abuse, providing no documentation on the use of methamphetamine in standardized settings. Additionally, the underlying molecular mechanism on how methamphetamine affects the cardiovascular system remains elusive. A relationship was sought between cardiotoxicity and arrhythmia with associated methamphetamine abuse in zebrafish to identify and to understand the adverse cardiac symptoms associated with methamphetamine. Zebrafish were first treated with methamphetamine 3 times a week over a 2-week duration. Immediately after treatment, zebrafish underwent electrocardiogram (ECG) measurement using an in-house developed acquisition system for electrophysiological analysis. Subsequent analyses of cAMP expression and Ca2+ regulation in zebrafish cardiomyocytes were conducted. cAMP is vital to development of myocardial fibrosis and arrhythmia, prominent symptoms in the development of cardiovascular diseases. Ca2+ dysregulation is also a factor in inducing arrhythmias. During the first week of treatment, zebrafish that were administered with methamphetamine displayed a decrease in heart rate, which persisted throughout the second week and remained significantly lower than the heart rate of untreated fish. Results also indicate an increased heart rate variability during the early stage of treatment followed by a decrease in the late stage for methamphetamine-treated fish over the duration of the experiment, suggesting a biphasic response to methamphetamine exposure. Methamphetamine-treated fish also exhibited reduced QTc intervals throughout the experiment. Results from the cAMP and Ca2+ assays demonstrate that cAMP was upregulated and Ca2+ was dysregulated in response to methamphetamine treatment. Collagenic assays indicated significant fibrotic response to methamphetamine treatment. These results provide potential insight into the role of methamphetamine in the development of fibrosis and arrhythmia due to downstream effectors of cAMP.

Acquirement of the autonomic nervous system modulation evaluated by heart rate variability in medaka (Oryzias latipes)

Small teleosts have recently been established as models of human diseases. However, measuring heart rate by electrocardiography is highly invasive for small fish and not widely used. The physiological nature and function of vertebrate autonomic nervous system (ANS) modulation of the heart has traditionally been investigated in larvae, transparent but with an immature ANS, or in anesthetized adults, whose ANS activity may possibly be disturbed under anesthesia. Here, we defined the frequency characteristics of heart rate variability (HRV) modulated by the ANS from observations of heart movement in high-speed movie images and changes in ANS regulation under environmental stimulation in unanesthetized adult medaka (Oryzias latipes). The HRV was significantly reduced by atropine (1 mM) in the 0.25-0.65 Hz and by propranolol (100 μM) at 0.65-1.25 Hz range, suggesting that HRV in adult medaka is modulated by both the parasympathetic and sympathetic nervous systems within these frequency ranges. Such modulations of HRV by the ANS in adult medaka were remarkably suppressed under anesthesia and continuous exposure to light suppressed HRV only in the 0.25-0.65 Hz range, indicating parasympathetic withdrawal. Furthermore, pre-hatching embryos did not show HRV and the power of HRV developed as fish grew. These results strongly suggest that ANS modulation of the heart in adult medaka is frequency-dependent phenomenon, and that the impact of long-term environmental stimuli on ANS activities, in addition to development of ANS activities, can be precisely evaluated in medaka using the presented method.

P66Shc (Shc1) Zebrafish Mutant Line as a Platform for Testing Decreased Reactive Oxygen Species in Pathology

Reactive oxygen species (ROS) dysregulation exacerbates many pathologies but must remain within normal ranges to maintain cell function. Since ROS-mediated pathology and routine cell function are coupled, in vivo models evaluating low-ROS background effects on pathology are limited. Some models alter enzymatic antioxidant expression/activity, while others involve small molecule antioxidant administration. These models cause non-specific ROS neutralization, decreasing both beneficial and detrimental ROS. This is detrimental in cardiovascular pathology, despite the negative effects excessive ROS has on these pathologies. Thus, current trends in ROS-mediated pathology have shifted toward selective inhibition of ROS producers that are dysregulated during pathological insults, such as p66Shc. In this study, we evaluated a zebrafish heterozygote p66Shc hypomorphic mutant line as a low-ROS myocardial infarction (MI) pathology model that mimics mammalian MI. Our findings suggest this zebrafish line does not have an associated negative phenotype, but has decreased body mass and tissue ROS levels that confer protection against ROS-mediated pathology. Therefore, this line may provide a low-ROS background leading to new insights into disease.

The Effect of Hypothermia and Osmotic Shock on the Electrocardiogram of Adult Zebrafish

Simple Summary

Assessing cardiac toxicity of new drugs is a requirement for their approval. One of the parameters which is carefully looked at is the QT interval, which is determined using an electrocardiogram (ECG). Before undertaking clinical trials using human patients, it is important to first perform pre-clinical tests using animal models. Zebrafish are widely used to study cardiac physiology and several reports suggest that although ECG measurement can be performed, the recording configuration appears to affect the results. Our research aimed to provide a comprehensive characterization of adult zebrafish ECG to determine the best practice for using this model during cardiac toxicity trials. We tested three recording configurations and determined that exposing the heart provided the most reliable and reproducible ECG recordings. We also determined the most accurate correction to apply to calculate the corrected QT, which makes the QT interval independent of the heart rate, a critical parameter when assessing drug cardiac toxicity. Overall, our study highlights the best conditions to record zebrafish ECG and demonstrates their utility for cardiac toxicity testing.

Abstract

The use of zebrafish to explore cardiac physiology has been widely adopted within the scientific community. Whether this animal model can be used to determine drug cardiac toxicity via electrocardiogram (ECG) analysis is still an ongoing question. Several reports indicate that the recording configuration severely affects the ECG waveforms and its derived-parameters, emphasizing the need for improved characterization. To address this problem, we recorded ECGs from adult zebrafish hearts in three different configurations (unexposed heart, exposed heart, and extracted heart) to identify the most reliable method to explore ECG recordings at baseline and in response to commonly used clinical therapies. We found that the exposed heart configuration provided the most reliable and reproducible ECG recordings of waveforms and intervals. We were unable to determine T wave morphology in unexposed hearts. In extracted hearts, ECG intervals were lengthened and P waves were unstable. However, in the exposed heart configuration, we were able to reliably record ECGs and subsequently establish the QT-RR relationship (Holzgrefe correction) in response to changes in heart rate.

A novel wireless ECG system for prolonged monitoring of multiple zebrafish for heart disease and drug screening studies

Zebrafish and their mutant lines have been extensively used in cardiovascular studies. In the current study, the novel system, Zebra II, is presented for prolonged electrocardiogram (ECG) acquisition and analysis for multiple zebrafish within controllable working environments. The Zebra II is composed of a perfusion system, apparatuses, sensors, and an in-house electronic system. First, the Zebra II is validated in comparison with a benchmark system, namely iWORX, through various experiments. The validation displayed comparable results in terms of data quality and ECG changes in response to drug treatment. The effects of anesthetic drugs and temperature variation on zebrafish ECG were subsequently investigated in experiments that need real-time data assessment. The Zebra II's capability of continuous anesthetic administration enabled prolonged ECG acquisition up to 1 h compared to that of 5 min in existing systems. The novel, cloud-based, automated analysis with data obtained from four fish further provided a useful solution for combinatorial experiments and helped save significant time and effort. The system showed robust ECG acquisition and analytics for various applications including arrhythmia in sodium induced sinus arrest, temperature-induced heart rate variation, and drug-induced arrhythmia in Tg(SCN5A-D1275N) mutant and wildtype fish. The multiple channel acquisition also enabled the implementation of randomized controlled trials on zebrafish models. The developed ECG system holds promise and solves current drawbacks in order to greatly accelerate drug screening applications and other cardiovascular studies using zebrafish.

Ultrasonic technologies in imaging and drug delivery

Ultrasonic technologies show great promise for diagnostic imaging and drug delivery in theranostic applications. The development of functional and molecular ultrasound imaging is based on the technical breakthrough of high frame-rate ultrasound. The evolution of shear wave elastography, high-frequency ultrasound imaging, ultrasound contrast imaging, and super-resolution blood flow imaging are described in this review. Recently, the therapeutic potential of the interaction of ultrasound with microbubble cavitation or droplet vaporization has become recognized. Microbubbles and phase-change droplets not only provide effective contrast media, but also show great therapeutic potential. Interaction with ultrasound induces unique and distinguishable biophysical features in microbubbles and droplets that promote drug loading and delivery. In particular, this approach demonstrates potential for central nervous system applications. Here, we systemically review the technological developments of theranostic ultrasound including novel ultrasound imaging techniques, the synergetic use of ultrasound with microbubbles and droplets, and microbubble/droplet drug-loading strategies for anticancer applications and disease modulation. These advancements have transformed ultrasound from a purely diagnostic utility into a promising theranostic tool.

Development and optimization of an in vivo electrocardiogram recording method and analysis program for adult zebrafish

Clinically pertinent electrocardiogram (ECG) data from model systems, such as zebrafish, are crucial for illuminating factors contributing to human cardiac electrophysiological abnormalities and disease. Current zebrafish ECG collection strategies have not adequately addressed the consistent acquisition of high-quality traces or sources of phenotypic variation that could obscure data interpretation. Thus, we developed a novel platform to ensure high-quality recording of in vivo subdermal adult zebrafish ECGs and zebrafish ECG reading GUI (zERG), a program to acquire measurements from traces that commercial software cannot examine owing to erroneous peak calling. We evaluate normal ECG trait variation, revealing highly reproducible intervals and wave amplitude variation largely driven by recording artifacts, and identify sex and body size as potential confounders to PR, QRS and QT intervals. With this framework, we characterize the effect of the class I anti-arrhythmic drug flecainide acetate on adults, provide support for the impact of a Long QT syndrome model, and establish power calculations for this and other studies. These results highlight our pipeline as a robust approach to evaluate zebrafish models of human cardiac electrophysiological phenotypes.

Constructing Adult Zebrafish Einthoven's Triangle to Define Electrical Heart Axes

Zebrafish is a popular high-throughput vertebrate model to study human cardiac electrophysiology, arrhythmias, and myopathies. One reason for this popularity is the purported striking similarities between zebrafish and human electrocardiograms (ECGs). However, zebrafish electrical heart axes were unknown. It is impossible to define heart axis based on single-lead ECG because determination of an electrical heart axis in the frontal plane requires the use of the hexaxial reference system (or Cabrera system) derived from Einthoven’s triangle. Construction of Einthoven’s triangle requires simultaneous ECG recording from at least two Einthoven bipolar leads. Therefore, we systematically constructed the first zebrafish Einthoven’s triangle by simultaneous bipolar dual-lead ECG recording to determine for the first time the three frontal electrical heart axes using the Cabrera system. Comparing zebrafish with human Einthoven’s triangle reveals that their normal frontal electrical axes were reflections of each other across 0° in the Cabrera system. The responsible mechanisms involve zebrafish vs. human cardiac activation propagating in the same direction along the heart horizontal axis but in opposite directions along the heart longitudinal axis. The same observations are true for zebrafish vs. human cardiac repolarization. This study marks a technical breakthrough in the first bipolar dual-lead ECG recording in live adult zebrafish to construct for the first time zebrafish Einthoven’s triangle. This first systematic analysis of the actual differences and similarities between normal adult zebrafish and human Einthoven’s triangles unmasked differences and similarities in the underlying cardiac axis mechanisms. Insights of the live adult zebrafish main heart axis and its three frontal electrical heart axes provide critical contextual framework to interpret the clinical relevance of the adult zebrafish heart as model for human cardiac electrophysiology.

Endothelial mechanotransduction in cardiovascular development and regeneration: emerging approaches and animal models

Living cells are exposed to multiple mechanical stimuli from the extracellular matrix or from surrounding cells. Mechanoreceptors are molecules that display status changes in response to mechanical stimulation, transforming physical cues into biological responses to help the cells adapt to dynamic changes of the microenvironment. Mechanical stimuli are responsible for shaping the tridimensional development and patterning of the organs in early embryonic stages. The development of the heart is one of the first morphogenetic events that occur in embryos. As the circulation is established, the vascular system is exposed to constant shear stress, which is the force created by the movement of blood. Both spatial and temporal variations in shear stress differentially modulate critical steps in heart development, such as trabeculation and compaction of the ventricular wall and the formation of the heart valves. Zebrafish embryos are small, transparent, have a short developmental period and allow for real-time visualization of a variety of fluorescently labeled proteins to recapitulate developmental dynamics. In this review, we will highlight the application of zebrafish models as a genetically tractable model for investigating cardiovascular development and regeneration. We will introduce our approaches to manipulate mechanical forces during critical stages of zebrafish heart development and in a model of vascular regeneration, as well as advances in imaging technologies to capture these processes at high resolution. Finally, we will discuss the role of molecules of the Plexin family and Piezo cation channels as major mechanosensors recently implicated in cardiac morphogenesis.

A Systematic Exposition of Methods used for Quantification of Heart Regeneration after Apex Resection in Zebrafish

Myocardial infarction (MI) is a worldwide condition that affects millions of people. This is mainly caused by the adult human heart lacking the ability to regenerate upon injury, whereas zebrafish have the capacity through cardiomyocyte proliferation to fully regenerate the heart following injury such as apex resection (AR). But a systematic overview of the methods used to evidence heart regrowth and regeneration in the zebrafish is lacking. Herein, we conducted a systematical search in Embase and Pubmed for studies on heart regeneration in the zebrafish following injury and identified 47 AR studies meeting the inclusion criteria. Overall, three different methods were used to assess heart regeneration in zebrafish AR hearts. 45 out of 47 studies performed qualitative (37) and quantitative (8) histology, whereas immunohistochemistry for various cell cycle markers combined with cardiomyocyte specific proteins was used in 34 out of 47 studies to determine cardiomyocyte proliferation qualitatively (6 studies) or quantitatively (28 studies). For both methods, analysis was based on selected heart sections and not the whole heart, which may bias interpretations. Likewise, interstudy comparison of reported cardiomyocyte proliferation indexes seems complicated by distinct study designs and reporting manners. Finally, six studies performed functional analysis to determine heart function, a hallmark of human heart injury after MI. In conclusion, our data implies that future studies should consider more quantitative methods eventually taking the 3D of the zebrafish heart into consideration when evidencing myocardial regrowth after AR. Furthermore, standardized guidelines for reporting cardiomyocyte proliferation and sham surgery details may be considered to enable inter study comparisons and robustly determine the effect of given genes on the process of heart regeneration.

High-frequency ultrasound deformation imaging for adult zebrafish during heart regeneration

Background

The adult human heart cannot efficiently generate new cardiac muscle cells in response to injury, and, therefore, cardiac injury results in irreversible damage to cardiac functions. The zebrafish (Danio rerio) is a crucial animal model in cardiac research because of its remarkable capacity for tissue regeneration. An adult zebrafish can completely regenerate cardiac tissue without a scar being formed, even after 20% of its ventricular myocardium has been resected. Zebrafish have been utilized in developmental biology and genetics research; however, the details of myocardium motions during their cardiac cycle in different regeneration phases are still not fully understood.

Methods

In this study, we used a 70-MHz high-resolution ultrasound deformation imaging system to observe the functional recovery of zebrafish hearts after amputation of the ventricular apex.

Results

The myocardial deformation and cardiac output (CO) were measured in different regeneration phases relative to the day of amputation. In response to the damage to the heart, the peak systolic strain (ε_max) and strain during ejection time (ε_ej) were lower than normal at 3 days after the myocardium amputation. The CO had normalized to the baseline values at 7 days after surgery.

Conclusions

Our results confirm that the imaging system constructed for this study is suitable for examining zebrafish cardiac functions during heart regeneration.

Development of a Simple ImageJ-Based Method for Dynamic Blood Flow Tracking in Zebrafish Embryos and Its Application in Drug Toxicity Evaluation

This study aimed to develop a simple and cost-effective method to measure blood flow in zebrafish by using an image-based approach. Three days post fertilization (dpf) zebrafish embryos were mounted with methylcellulose and subjected to video recording for tracking blood flow under an inverted microscope equipped with a high-speed CCD camera. In addition, Hoffman lens was used to enhance the blood cell contrast. The red blood cell movement was tracked by using the TrackMate plug-in in the ImageJ image processing program. Moreover, Stack Difference and Time Series Analyzer plug-in were used to detect dynamic pixel changes over time to calculate the blood flow rate. In addition to blood flow velocity and heart rate, the effect of drug treatments on other cardiovascular function parameters, such as stroke volume and cardiac output remains to be explored. Therefore, by using this method, the potential side effects on the cardiovascular performance of ethyl 3-aminobenzoate methanesulfonate (MS222) and 3-isobutyl-1-methylxanthine (IBMX) were evaluated. MS222 is a common anesthetic, while IBMX is a naturally occurring methylxanthine. Compared to normal embryos, MS222- and IBMX-treated embryos had a reduced blood flow velocity by approximately 72% and 58%, respectively. This study showed that MS222 significantly decreased the heart rate, whereas IBMX increased the heart rate. Moreover, it also demonstrated that MS222 treatment reduced 50% of the stroke volume and cardiac output. While IBMX decreased the stroke volume only. The results are in line with previous studies that used expensive instruments and complicated software analysis to assess cardiovascular function. In conclusion, a simple and low-cost method can be used to study blood flow in zebrafish embryos for compound screening. Furthermore, it could provide a precise measurement of clinically relevant cardiac functions, specifically heart rate, stroke volume, and cardiac output.

Acquisition, Processing and Analysis of Electrocardiogram in Awake Zebrafish

Long-term monitoring of intrinsic electrocardiogram (ECG) in zebrafish plays a crucial role in heart disease studies as well as drug screening. In this work, we developed a polymer-based apparatus with embedded flexible thin-film electrodes to acquire ECG signals of awake zebrafish. The apparatus was made of polydimethylsiloxane (PDMS) using the molding technique with molds formed by 3D printing. A graphical user interface (GUI) was built in National Instruments LabView platform for real-time recording, processing and analysis. The program provided important features, such as signal de-noising, characteristic wave detection and anomaly detection. Further, it could operate on both real-time coming signals as well as previously-saved data, facilitating analysis and interpretation. We demonstrated the use of our system to investigate the effects of the anesthetic drug, namely Tricaine (MS-222), on cardiac electrophysiology of zebrafish, revealing promising findings. We speculate that our novel system may contribute to a host of studies in various disciplines using the zebrafish model.

Development of a rapid and economic in vivo electrocardiogram platform for cardiovascular drug assay and electrophysiology research in adult zebrafish

Zebrafish is a popular and favorable model organism for cardiovascular research, with an increasing number of studies implementing functional assays in the adult stage. For example, the application of electrocardiography (ECG) in adult zebrafish has emerged as an important tool for cardiac pathophysiology, toxicity, and chemical screen studies. However, few laboratories are able to perform such functional analyses due to the high cost and limited availability of a convenient in vivo ECG recording system. In this study, an inexpensive ECG recording platform and operation protocol that has been optimized for adult zebrafish ECG research was developed. The core hardware includes integration of a ready-to-use portable ECG kit with a set of custom-made needle electrode probes. A combined anesthetic formula of MS-222 and isoflurane was first tested to determine the optimal assay conditions to minimize the interference to zebrafish cardiac physiology under sedation. For demonstration, we treated wild-type zebrafish with different pharmacological agents known to affect cardiac rhythms in humans. Conserved electrophysiological responses to these drugs were induced in adult zebrafish and recorded in real time. This economic ECG platform has the potential to facilitate teaching and training in cardiac electrophysiology with adult zebrafish and to promote future translational applications in cardiovascular medicine.

Advanced microscopy to elucidate cardiovascular injury and regeneration: 4D light-sheet imaging

The advent of 4-dimensional (4D) light-sheet fluorescence microscopy (LSFM) has provided an entry point for rapid image acquisition to uncover real-time cardiovascular structure and function with high axial resolution and minimal photo-bleaching/-toxicity. We hereby review the fundamental principles of our LSFM system to investigate cardiovascular morphogenesis and regeneration after injury. LSFM enables us to reveal the micro-circulation of blood cells in the zebrafish embryo and assess cardiac ventricular remodeling in response to chemotherapy-induced injury using an automated segmentation approach. Next, we review two distinct mechanisms underlying zebrafish vascular regeneration following tail amputation. We elucidate the role of endothelial Notch signaling to restore vascular regeneration after exposure to the redox active ultrafine particles (UFP) in air pollutants. By manipulating the blood viscosity and subsequently, endothelial wall shear stress, we demonstrate the mechanism whereby hemodynamic shear forces impart both mechanical and metabolic effects to modulate vascular regeneration. Overall, the implementation of 4D LSFM allows for the elucidation of mechanisms governing cardiovascular injury and regeneration with high spatiotemporal resolution.

Atrium-specific ion channels in the zebrafish-A role of IKACh in atrial repolarization

AIM

The zebrafish has emerged as a novel model for investigating cardiac physiology and pathology. The aim of this study was to investigate the atrium-specific ion channels responsible for shaping the atrial cardiac action potential in zebrafish.

METHODS

Using quantitative polymerase chain reaction, we assessed the expression level of atrium-specific potassium channels. The functional role of these channels was studied by patch clamp experiments on isolated atrial and ventricular cardiomyocytes and by optical mapping of explanted adult zebrafish hearts. Finally, surface ECGs were recorded to establish possible in vivo roles of atrial ion channels.

RESULTS

In isolated adult zebrafish hearts, we identified the expression of kcnk3, kcnk9, kcnn1, kcnn2, kcnn3, kcnj3 and kcnj5, the genes that encode the atrium-specific K_2P , K_Ca 2.x and K_ir 3.1/4 (K_ACh ) ion channels. The electrophysiological data indicate that the acetylcholine-activated inward-rectifying current, I_KACh, plays a major role in the zebrafish atrium, whereas K_2P 3.1/9.1 and K_Ca 2.x channels do not appear to be involved in regulating the action potential in the zebrafish heart.

CONCLUSION

We demonstrate that the acetylcholine-activated inward-rectifying current (I_KACh ) current plays a major role in the zebrafish atrium and that the zebrafish could potentially be a cost-effective and reliable model for pharmacological testing of atrium-specific I_KACh modulating compounds.

Cardiac voltage-gated sodium channel expression and electrophysiological characterization of the sodium current in the zebrafish (Danio rerio) ventricle

Na⁺ channel α-subunit composition of the zebrafish heart and electrophysiological properties of Na⁺ current (I_Na) of zebrafish ventricular myocytes were examined. Eight Na⁺ channel α-subunits were expressed in both atrium and ventricle of the zebrafish heart. Na_v1.5Lb, an orthologue to the human Na_v1.5, was clearly the predominant isoform in both chambers representing 65.2 ± 4.1% and 83.1 ± 2.1% of all Na⁺ channel transcripts in atrium and ventricle, respectively. Na_v1.4b, an orthologue to human Na_v1.4, formed 34.1 ± 4.1 and 16.2 ± 2.0% of the Na⁺ channel transcripts in atrium and ventricle, respectively. The density of I_Na and the rate of action potential upstroke in zebrafish ventricular myocytes at 28 °C were similar to those of human ventricles at the comparable temperature. Na⁺ channel isoforms and the main electrophysiological characteristics of the I_Na are largely similar in zebrafish and human hearts indicating evolutionary conservation of Na⁺ channel composition and function. The zebrafish I_Na differs from the human cardiac I_Na in terms of higher tetrodotoxin sensitivity (IC₅₀-value = 5.3 ± 0.1 nM) and slower inactivation kinetics. The zebrafish I_Na was inhibited with tricaine (MS-222) with an IC₅₀-value of 1.2 ± 0.18 mM (336 mg l^-1), suggesting some care in the use of MS-222 as an anesthetic.

Cadmium stress assessment based on the electrocardiogram characteristics of zebra fish (Danio rerio): QRS complex could play an important role

The electrocardiogram (ECG) of zebra fish (Danio rerio) expresses cardiac features that are similar to humans. Here we use sharp microelectrode measurements to obtain ECG characteristics in adult zebra fish and analyze the effects of cadmium chloride (CdCl₂) on the heart. We observe the overall changes of ECG parameters in different treatments (0.1 TU, 0.5 TU and 1.0 TU CdCl₂), including P wave, Q wave, R wave, S wave, T wave, PR interval (atrial contraction), QRS complex (ventricular depolarization), ST segment, and QT interval (ventricular repolarization). The trends of the ECG parameters showed some responses to the concentration and exposure time of CdCl₂, but it was difficult to obtain more information about the useful indicators in water quality assessment depending on tendency analysis alone. A self-organizing map (SOM) showed that P values, R values, and T values were similar; R wave and T wave amplitude were similar; and most important, QRS value was similar to the CdCl₂ stress according to the classified data patterns including CdCl₂ stress (E) and ECG components based on the Ward linkage. It suggested that the duration of QRS complex was related to environmental stress E directly. The specification and evaluation of ECG parameters in Cd²⁺ pollution suggested that there is a markedly significant correlation between QRS complex and CdCl₂ stress with the highest r (0.729) and the smallest p (0.002) among all ECG characteristics. In this case, it is concluded that QRS complex can be used as an indicator in the CdCl₂ stress assessment due to the lowest AIC data abased on the linear regression model between the CdCl₂ stress and ECG parameters.

Automated Segmentation of Light-Sheet Fluorescent Imaging to Characterize Experimental Doxorubicin-Induced Cardiac Injury and Repair

This study sought to develop an automated segmentation approach based on histogram analysis of raw axial images acquired by light-sheet fluorescent imaging (LSFI) to establish rapid reconstruction of the 3-D zebrafish cardiac architecture in response to doxorubicin-induced injury and repair. Input images underwent a 4-step automated image segmentation process consisting of stationary noise removal, histogram equalization, adaptive thresholding, and image fusion followed by 3-D reconstruction. We applied this method to 3-month old zebrafish injected intraperitoneally with doxorubicin followed by LSFI at 3, 30, and 60 days post-injection. We observed an initial decrease in myocardial and endocardial cavity volumes at day 3, followed by ventricular remodeling at day 30, and recovery at day 60 (P < 0.05, n = 7-19). Doxorubicin-injected fish developed ventricular diastolic dysfunction and worsening global cardiac function evidenced by elevated E/A ratios and myocardial performance indexes quantified by pulsed-wave Doppler ultrasound at day 30, followed by normalization at day 60 (P < 0.05, n = 9-20). Treatment with the γ-secretase inhibitor, DAPT, to inhibit cleavage and release of Notch Intracellular Domain (NICD) blocked cardiac architectural regeneration and restoration of ventricular function at day 60 (P < 0.05, n = 6-14). Our approach provides a high-throughput model with translational implications for drug discovery and genetic modifiers of chemotherapy-induced cardiomyopathy.

LITTLE FISH, BIG DATA: ZEBRAFISH AS A MODEL FOR CARDIOVASCULAR AND METABOLIC DISEASE

The burden of cardiovascular and metabolic diseases worldwide is staggering. The emergence of systems approaches in biology promises new therapies, faster and cheaper diagnostics, and personalized medicine. However, a profound understanding of pathogenic mechanisms at the cellular and molecular levels remains a fundamental requirement for discovery and therapeutics. Animal models of human disease are cornerstones of drug discovery as they allow identification of novel pharmacological targets by linking gene function with pathogenesis. The zebrafish model has been used for decades to study development and pathophysiology. More than ever, the specific strengths of the zebrafish model make it a prime partner in an age of discovery transformed by big-data approaches to genomics and disease. Zebrafish share a largely conserved physiology and anatomy with mammals. They allow a wide range of genetic manipulations, including the latest genome engineering approaches. They can be bred and studied with remarkable speed, enabling a range of large-scale phenotypic screens. Finally, zebrafish demonstrate an impressive regenerative capacity scientists hope to unlock in humans. Here, we provide a comprehensive guide on applications of zebrafish to investigate cardiovascular and metabolic diseases. We delineate advantages and limitations of zebrafish models of human disease and summarize their most significant contributions to understanding disease progression to date.

Optimal Anesthetic Regime for Motionless Three-Dimensional Image Acquisition During Longitudinal Studies of Adult Nonpigmented Zebrafish

With many live imaging techniques, it is crucial that a deep level of anesthesia is reached and maintained throughout image acquisition without reducing zebrafish viability. This is particularly true for three-dimensional tomographic imaging modalities. Currently, the most commonly used anesthetic in the zebrafish community, MS-222 (tricaine methanesulfonate), does not allow this. We show, using a combination of both MS-222 and isoflurane, that we can significantly improve the anesthetic regime required for motionless image acquisition of live adult zebrafish. We have benchmarked this against the requirements of our novel quantitative imaging platform, compressive sensing optical projection tomography. Using nonpigmented transgenic zebrafish, we show that a combination of 175 ppm of both anesthetics improves the maintenance of deep anesthesia for prolonged periods of time and it can be used repeatedly to enable longitudinal imaging. Importantly, it does not affect the health or viability of the adult zebrafish. We also show that nonpigmented fish, with a mutated form of the gene transparent, took significantly longer to reach deep anesthesia. The anesthetic regime presented in this study should lead to significant improvements in accuracy and information achievable from imaging live adult zebrafish and in its application to longitudinal studies.

3D Finite Element Electrical Model of Larval Zebrafish ECG Signals

Assessment of heart function in zebrafish larvae using electrocardiography (ECG) is a potentially useful tool in developing cardiac treatments and the assessment of drug therapies. In order to better understand how a measured ECG waveform is related to the structure of the heart, its position within the larva and the position of the electrodes, a 3D model of a 3 days post fertilisation (dpf) larval zebrafish was developed to simulate cardiac electrical activity and investigate the voltage distribution throughout the body. The geometry consisted of two main components; the zebrafish body was modelled as a homogeneous volume, while the heart was split into five distinct regions (sinoatrial region, atrial wall, atrioventricular band, ventricular wall and heart chambers). Similarly, the electrical model consisted of two parts with the body described by Laplace's equation and the heart using a bidomain ionic model based upon the Fitzhugh-Nagumo equations. Each region of the heart was differentiated by action potential (AP) parameters and activation wave conduction velocities, which were fitted and scaled based on previously published experimental results. ECG measurements in vivo at different electrode recording positions were then compared to the model results. The model was able to simulate action potentials, wave propagation and all the major features (P wave, R wave, T wave) of the ECG, as well as polarity of the peaks observed at each position. This model was based upon our current understanding of the structure of the normal zebrafish larval heart. Further development would enable us to incorporate features associated with the diseased heart and hence assist in the interpretation of larval zebrafish ECGs in these conditions.

Improvement of surface ECG recording in adult zebrafish reveals that the value of this model exceeds our expectation

The adult zebrafish has been used to model the electrocardiogram (ECG) for human cardiovascular studies. Nonetheless huge variations are observed among studies probably because of the lack of a reliable and reproducible recording method. In our study, an adult zebrafish surface ECG recording technique was improved using a multi-electrode method and by pre-opening the pericardial sac. A convenient ECG data analysis method without wavelet transform was also established. Intraperitoneal injection of KCl in zebrafish induced an arrhythmia similar to that of humans, and the arrhythmia was partially rescued by calcium gluconate. Amputation and cryoinjury of the zebrafish heart induced ST segment depression and affected QRS duration after injury. Only cryoinjury decelerated the heart rate. Different changes were also observed in the QT interval during heart regeneration in these two injury models. We also characterized the electrocardiophysiology of breakdance zebrafish mutant with a prolonged QT interval, that has not been well described in previous studies. Our study provided a reliable and reproducible means to record zebrafish ECG and analyse data. The detailed characterization of the cardiac electrophysiology of zebrafish and its mutant revealed that the potential of the zebrafish in modeling the human cardiovascular system exceeds expectations.

Modeling GATAD1-Associated Dilated Cardiomyopathy in Adult Zebrafish

Animal models have played a critical role in validating human dilated cardiomyopathy (DCM) genes, particularly those that implicate novel mechanisms for heart failure. However, the disease phenotype may be delayed due to age-dependent penetrance. For this reason, we generated an adult zebrafish model, which is a simpler vertebrate model with higher throughput than rodents. Specifically, we studied the zebrafish homologue of GATAD1, a recently identified gene for adult-onset autosomal recessive DCM. We showed cardiac expression of gatad1 transcripts, by whole mount in situ hybridization in zebrafish embryos, and demonstrated nuclear and sarcomeric I-band subcellular localization of Gatad1 protein in cardiomyocytes, by injecting a Tol2 plasmid encoding fluorescently-tagged Gatad1. We next generated gatad1 knock-out fish lines by TALEN technology and a transgenic fish line that expresses the human DCM GATAD1-S102P mutation in cardiomyocytes. Under stress conditions, longitudinal studies uncovered heart failure (HF)-like phenotypes in stable KO mutants and a tendency toward HF phenotypes in transgenic lines. Based on these efforts of studying a gene-based inherited cardiomyopathy model, we discuss the strengths and bottlenecks of adult zebrafish as a new vertebrate model for assessing candidate cardiomyopathy genes.

Hemodynamics and ventricular function in a zebrafish model of injury and repair

Myocardial infarction results in scar tissue and irreversible loss of ventricular function. Unlike humans, zebrafish has the capacity to remove scar tissue after injury. To assess ventricular function during repair, we synchronized microelectrocardiogram (μECG) signals with a high-frequency ultrasound pulsed-wave (PW) Doppler to interrogate cardiac hemodynamics. μECG signals allowed for identification of PW Doppler signals for passive (early [E]-wave velocity) and active ventricular filling (atrial [A]-wave velocity) during diastole. The A wave (9.0±1.2 cm·s(-1)) is greater than the E wave (1.1±0.4 cm·s(-1)), resulting in an E/A ratio <1 (0.12±0.05, n=6). In response to cryocauterization to the ventricular epicardium, the E-wave velocity increased, accompanied by a rise in the E/A ratio at 3 days postcryocauterization (dpc) (0.55±0.13, n=6, p<0.001 vs. sham). The E waves normalize toward the baseline, along with a reduction in the E/A ratio at 35 dpc (0.36±0.06, n=6, p<0.001 vs. sham) and 65 dpc (0.2±0.16, n=6, p<0.001 vs. sham). In zebrafish, E/A<1 at baseline is observed, suggesting the distinct two-chamber system in which the pressure gradient across the atrioventricular valve is higher compared with the ventriculobulbar valve. The initial rise and subsequent normalization of E/A ratios support recovery in the ventricular diastolic function.

The effects of cocaine on heart rate and electrocardiogram in zebrafish (Danio rerio)

Zebrafish (Danio rerio) have been used as a model organism to explore the genetic basis for responsiveness to addictive drugs like cocaine. However, very little is known about how the physiological response to cocaine is mediated in zebrafish. In the present study electrocardiograms (ECGs) were recorded from adult zebrafish treated with cocaine. Treatment with cocaine resulted in a bell-shaped dose response curve with a maximal change in heart rate seen using 5mg/L cocaine. Higher doses resulted in a higher percentage of fish showing bradycardia. The cocaine-induced tachycardia was blocked by co-treatment with propranolol, a β-adrenergic antagonist, but potentiated by co-treatment with phentolamine, an α-adrenergic antagonist. Co-treatment with atropine, a classic cholinergic antagonist, had no effect on cocaine-induced tachycardia. Cocaine treatment of adult fish changed the ECG of treated fish, inducing a dose-dependent increase in QT interval after adjusting for heart rate (QTc), while not affecting the PR or QRS intervals. The acute effects of cocaine on heart rate were examined in 5-day old embryos to see if zebrafish might serve as a suitable model organism to study possible links of embryonic physiological response to subsequent adult behavioral response to the drug. Cocaine treatment of 5-day old zebrafish embryos also resulted in a bell-shaped dose response curve, with maximal tachycardia achieved with 10mg/L. The response in embryonic fish was thus comparable to that in adults and raises the possibility that the effects of embryonic exposure to cocaine on the developing cardiovascular system can be effectively modeled in zebrafish.

Electrical and Mechanical Strategies to Enable Cardiac Repair and Regeneration

Inadequate replacement of lost ventricular myocardium from myocardial infarction leads to heart failure. Investigating the regenerative capacity of mammalian hearts represents an emerging direction for tissue engineering and cell-based therapy. Recent advances in stem cells hold promise to restore cardiac functions. However, embryonic or induced pluripotent stem cell-derived cardiomyocytes lack functional phenotypes of the native myocardium, and transplanted tissues are not fully integrated for synchronized electrical and mechanical coupling with the host. In this context, this review highlights the mechanical and electrical strategies to promote cardiomyocyte maturation and integration, and to assess the functional phenotypes of regenerating myocardium. Simultaneous microelectrocardiogram and high-frequency ultrasound techniques will also be introduced to assess electrical and mechanical coupling for small animal models of heart regeneration.

Wearable multi-channel microelectrode membranes for elucidating electrophysiological phenotypes of injured myocardium

Understanding the regenerative capacity of small vertebrate models has provided new insights into the plasticity of injured myocardium. Here, we demonstrate the application of flexible microelectrode arrays (MEAs) in elucidating electrophysiological phenotypes of zebrafish and neonatal mouse models of heart regeneration. The 4-electrode MEA membranes were designed to detect electrical signals in the aquatic environment. They were micro-fabricated to adhere to the non-planar body surface of zebrafish and neonatal mice. The acquired signals were processed to display an electrocardiogram (ECG) with high signal-to-noise-ratios, and were validated via the use of conventional micro-needle electrodes. The 4-channel MEA provided signal stability and spatial resolution, revealing the site-specific electrical injury currents such as ST-depression in response to ventricular cryo-injury. Thus, our polymer-based and wearable MEA membranes provided electrophysiological insights into long-term conduction phenotypes for small vertebral models of heart injury and regeneration with a translational implication for monitoring cardiac patients.

Flexible and waterproof micro-sensors to uncover zebrafish circadian rhythms: The next generation of cardiac monitoring for drug screening

Flexible electronics are the next generation of sensors for mobile health and implantation. Zebrafish (Danio rerio) is an emergent strategy for pre-clinical drug development and toxicity testing. To address the confounding effects from sedation of fish and removal from the aquatic habitat for micro-electrocardiogram (µECG) measurements, we developed waterproof and wearable sensors to uncover the circadian variation in heart rate (HR) and heart rate variability (HRV) (Massin et al., 2000). The parylene-C based ECG sensor consisted of an ultra-soft silicone integrated jacket designed to wrap around the fish during swimming. The Young's modulus of this silicone jacket matched with the fish surface, and an extended parylene cable connected the underwater chest electrodes with the out-of water electronics. In addition, embedded micro-glass spheres in the silicone effectively reduced the effective density of the jacket to ~1 g cm(-3). These innovations enabled physiological ECG telemetry in the fish's natural habitat without the need for sedation. Furthermore, a set of non-linear signal processing techniques filtered out the breathing and electromagnetic artifacts from the recorded signals. We observed a reduction in mean HR and an increase in HRV over 24h at 10 dpa, accompanied by QT prolongation as well as diurnal variations, followed by normalization in mean HR and QT intervals at 26 days post ventricular amputation (dpa). We revealed Amiodarone-mediated QTc prolongation, HR reduction and HRV increase otherwise masked by sedation. The novel features of the flexible silicon jacket for µECG telemetry unraveled the biological clock and normalization of QT intervals at 26 dpa, providing the first evidence of new physiological phenomena during cardiac injury and repair as well as cardiac drug-mediated aberrant rhythms. Thus, the light weight and waterproof design holds promise to advance the next generation of mobile health and drug discovery.

Dry-contact microelectrode membranes for wireless detection of electrical phenotypes in neonatal mouse hearts

Continuous monitoring of aberrant electrical rhythms during heart injury and repair requires prolonged data acquisition. We hereby developed a wearable microelectrode membrane that could be adherent to the chest of neonatal mice for in situ wireless recording of electrocardiogram (ECG) signals. The novel dry-contact membrane with a meshed parylene-C pad adjacent to the microelectrodes and the expandable meandrous strips allowed for varying size of the neonates. The performance was evaluated at the system level; specifically, the ECG signals (μV) acquired from the microelectrodes underwent two-stage amplification, band-pass filtering, and optical data transmission by an infrared Light Emitting Diode (LED) to the data-receiving unit. The circuitry was prototyped on a printed circuit board (PCB), consuming less than 300 μW, and was completely powered by an inductive coupling link. Distinct P waves, QRS complexes, and T waves of ECG signals were demonstrated from the non-pharmacologically sedated neonates at ~600 beats per minutes. Thus, we demonstrate the feasibility of both real-time and wireless monitoring cardiac rhythms in a neonatal mouse (17-20 mm and <1 g) via dry-contact microelectrode membrane; thus, providing a basis for diagnosing aberrant electrical conduction in animal models of cardiac injury and repair.

Central actions of serotonin and fluoxetine on the QT interval of the electrocardiogram in trout

QT interval of the electrocardiogram (ECG) is a measure of the duration of the ventricular depolarization and repolarization. In humans, prolongation of the QT interval is a known clinical risk factor for the development of ventricular arrhythmias including ‘Torsades de Pointes’ and possible sudden cardiac death. After oral administration, fluoxetine (FLX), as well as other selective serotonin (5-hydroxytryptamine, 5-HT) reuptake inhibitors can affect cardiac autonomic control, including the QT interval. However, the action of centrally administered FLX on the QT interval has never been explored. Consequently, using the unanesthetized trout as an animal model, we sought to compare the effects of intracerebroventricular (i.c.v.) injection of FLX (5, 15 or 25 µg) on the QT interval of the ECG with the effects observed following i.c.v. injection of 5-HT (0.05, 0.5 or 5 nmol). The QT interval was corrected for the R–R interval. The highest doses of centrally administered FLX and 5-HT induced a prolongation of the corrected QT (QTc) interval reaching a maximum value of 5–10 min after injection (+8% and +6% respectively, P < 0.05). The intra-arterial (i.a.) injections of 5-HT and FLX were without significant effect on the QTc. The i.a. injection of blockers of the autonomic nervous system indicated that the sympathetic nervous system modulated the QTc interval. In conclusion, our data demonstrate that for the first time in any animal species, cardiac electrophysiology is sensitive to central 5-HT and that FLX within the brain may disrupt the autonomic control of ventricular repolarization.

Prolonged in vivo imaging of Xenopus laevis

BACKGROUND

While live imaging of embryonic development over long periods of time is a well established method for embryos of the frog Xenopus laevis, once development has progressed to the swimming stages, continuous live imaging becomes more challenging because the tadpoles must be immobilized. Current imaging techniques for these advanced stages generally require bringing the tadpoles in and out of anesthesia for short imaging sessions at selected time points, severely limiting the resolution of the data.

RESULTS

Here we demonstrate that creating a constant flow of diluted tricaine methanesulfonate (MS-222) over a tadpole greatly improves their survival under anesthesia. Based on this result, we describe a new method for imaging stage 48 to 65 X. laevis, by circulating the anesthetic using a peristaltic pump. This supports the animal during continuous live imaging sessions for at least 48 hr. The addition of a stable optical window allows for high quality imaging through the anesthetic solution.

CONCLUSIONS

This automated imaging system provides for the first time a method for continuous observations of developmental and regenerative processes in advanced stages of Xenopus over 2 days. Developmental Dynamics 243:1011-1019, 2014. © 2014 Wiley Periodicals, Inc.

Optical assessment of the cardiac rhythm of contracting cardiomyocytes in vitro and a pulsating heart in vivo for pharmacological screening

Our quest in the pathogenesis and therapies targeting human heart diseases requires assessment of the contractile dynamics of cardiac models of varied complexity, such as isolated cardiomyocytes and the heart of a model animal. It is hence beneficial to have an integral means that can interrogate both cardiomyocytes in vitro and a heart in vivo. Herein we report an application of dual-beam optical reflectometry to determine noninvasively the rhythm of two representative cardiac models-chick embryonic cardiomyocytes and the heart of zebrafish. We probed self-beating cardiomyocytes and revealed the temporally varying contractile frequency with a short-time Fourier transform. Our unique dual-beam setup uniquely records the atrial and ventricular pulsations of zebrafish simultaneously. To minimize the cross talk between signals associated with atrial and ventricular chambers, we particularly modulated the two probe beams at distinct frequencies and extracted the signals specific to individual cardiac chambers with phase-sensitive detection. With this setup, we determined the atrio-ventricular interval, a parameter that is manifested by the electrical conduction from the atrium to the ventricle. To demonstrate pharmacological applications, we characterized zebrafish treated with various cardioactive and cardiotoxic drugs, and identified abnormal cardiac rhythms and atrioventricular (AV) blocks of varied degree. In light of its potential capability to assess cardiac models both in vitro and in vivo and to screen drugs with cardioactivity or toxicity, we expect this approach to have broad applications ranging from cardiopharmacology to developmental biology.

Zebrafish as a model of cardiac disease

The zebrafish has been rapidly adopted as a model for cardiac development and disease. The transparency of the embryo, its limited requirement for active oxygen delivery, and ease of use in genetic manipulations and chemical exposure have made it a powerful alternative to rodents. Novel technologies like TALEN/CRISPR-mediated genome engineering and advanced imaging methods will only accelerate its use. Here, we give an overview of heart development and function in the fish and highlight a number of areas where it is most actively contributing to the understanding of cardiac development and disease. We also review the current state of research on a feature that we only could wish to be conserved between fish and human; cardiac regeneration.

Atrial fibrillation pacing decreases intravascular shear stress in a New Zealand white rabbit model: implications in endothelial function

Atrial fibrillation (AF) is characterized by multiple rapid and irregular atrial depolarization, leading to rapid ventricular responses exceeding 100 beats per minute (bpm). We hypothesized that rapid and irregular pacing reduced intravascular shear stress (ISS) with implication to modulating endothelial responses. To simulate AF, we paced the left atrial appendage of New Zealand White rabbits (n = 4) at rapid and irregular intervals. Surface electrical cardiograms were recorded for atrial and ventricular rhythm, and intravascular convective heat transfer was measured by microthermal sensors, from which ISS was inferred. Rapid and irregular pacing decreased arterial systolic and diastolic pressures (baseline, 99/75 mmHg; rapid regular pacing, 92/73; rapid irregular pacing, 90/68; p < 0.001, n = 4), temporal gradients ([Formula: see text] from 1,275 ± 80 to 1,056 ± 180 dyne/cm(2) s), and reduced ISS (from baseline at 32.0 ± 2.4 to 22.7 ± 3.5 dyne/cm(2)). Computational fluid dynamics code demonstrated that experimentally inferred ISS provided a close approximation to the computed wall shear stress at a given catheter to vessel diameter ratio, shear stress range, and catheter position. In an in vitro flow system in which time-averaged shear stress was maintained at [Formula: see text] , we further demonstrated that rapid pulse rates at 150 bpm down-regulated endothelial nitric oxide, promoted superoxide (O 2 (.-) ) production, and increased monocyte binding to endothelial cells. These findings suggest that rapid pacing reduces ISS and [Formula: see text] , and rapid pulse rates modulate endothelial responses.

A transgenic zebrafish model of a human cardiac sodium channel mutation exhibits bradycardia, conduction-system abnormalities and early death

The recent exponential increase in human genetic studies due to the advances of next generation sequencing has generated unprecedented numbers of new gene variants. Determining which of these are causative of human disease is a major challenge. In-vitro studies and murine models have been used to study inherited cardiac arrhythmias but have several limitations. Zebrafish models provide an attractive alternative for modeling human heart disease due to similarities in cardiac electrophysiology and contraction, together with ease of genetic manipulation, external development and optical transparency. Although zebrafish cardiac mutants and morphants have been widely used to study loss and knockdown of zebrafish gene function, the phenotypic effects of human dominant-negative gene mutations expressed in transgenic zebrafish have not been evaluated. The aim of this study was to generate and characterize a transgenic zebrafish arrhythmia model harboring the pathogenic human cardiac sodium channel mutation SCN5A-D1275N, that has been robustly associated with a range of cardiac phenotypes, including conduction disease, sinus node dysfunction, atrial and ventricular arrhythmias, and dilated cardiomyopathy in humans and in mice. Stable transgenic fish with cardiac expression of human SCN5A were generated using Tol2-mediated transgenesis and cardiac phenotypes were analyzed using video microscopy and ECG. Here we show that transgenic zebrafish expressing the SCN5A-D1275N mutation, but not wild-type SCN5A, exhibit bradycardia, conduction-system abnormalities and premature death. We furthermore show that SCN5A-WT, and to a lesser degree SCN5A-D1275N, are able to compensate the loss of endogenous zebrafish cardiac sodium channels, indicating that the basic pathways, through which SCN5A acts, are conserved in teleosts. This proof-of-principle study suggests that zebrafish may be highly useful in vivo models to differentiate functional from benign human genetic variants in cardiac ion channel genes in a time- and cost-efficient manner. This article is part of a Special Issue entitled "Na(+) Regulation in Cardiac Myocytes".

Optimisation of embryonic and larval ECG measurement in zebrafish for quantifying the effect of QT prolonging drugs

Effective chemical compound toxicity screening is of paramount importance for safe cardiac drug development. Using mammals in preliminary screening for detection of cardiac dysfunction by electrocardiography (ECG) is costly and requires a large number of animals. Alternatively, zebrafish embryos can be used as the ECG waveform is similar to mammals, a minimal amount of chemical is necessary for drug testing, while embryos are abundant, inexpensive and represent replacement in animal research with reduced bioethical concerns. We demonstrate here the utility of pre-feeding stage zebrafish larvae in detection of cardiac dysfunction by electrocardiography. We have optimised an ECG recording system by addressing key parameters such as the form of immobilization, recording temperature, electrode positioning and developmental age. Furthermore, analysis of 3 days post fertilization (dpf) zebrafish embryos treated with known QT prolonging drugs such as terfenadine, verapamil and haloperidol led to reproducible detection of QT prolongation as previously shown for adult zebrafish. In addition, calculation of Z-factor scores revealed that the assay was sensitive and specific enough to detect large drug-induced changes in QTc intervals. Thus, the ECG recording system is a useful drug-screening tool to detect alteration to cardiac cycle components and secondary effects such as heart block and arrhythmias in zebrafish larvae before free feeding stage, and thus provides a suitable replacement for mammalian experimentation.

Optimization of the adult zebrafish ECG method for assessment of drug-induced QTc prolongation

INTRODUCTION

Recent studies have shown the utility of adult zebrafish ECG (electrocardiogram) in assessing drug-induced QTc prolongation. While the method has significant advantages over current ECG animal models including ethical issues, low compound requirement and expense, adoption of the method into drug discovery programs has been hampered by specific limitations. The limitations include the inability to determine the exact dose of test compound administered, and potential effects due to variables such as flow rate of oral perfusion and immobilization method. We describe a refined method for the reproducible recording of the adult zebrafish ECG and illustrate its application in investigating drug-induced QTc prolongation using the histamine receptor antagonist Terfenadine as a test drug.

METHOD

We chose to perform parenteral administration of test drug instead of perfusion on the basis of mg per kg body weight of adult zebrafish. Acclimatization and immobilization methods were optimized to avoid ECG artifacts due to sudden environmental changes. We further modified the formula for QT correction and ensured reproducible recording of stable ECGs. Various concentrations of Terfenadine were used and the resultant proarrhythmic effects were analyzed as compared to the baseline and untreated controls.

RESULTS

Normal, stable and reproducible ECGs were recorded in all zebrafish. Terfenadine at the rate of 0.1mg/kg body weight was found to be the NOAEL. We found an excellent correlation between known QTc effects in humans and those observed in adult zebrafish at all concentrations. All Terfenadine-induced proarrhythmic effects observed in zebrafish were dose and time dependent.

DISCUSSION

We report a refined method for reproducible recording of stable zebrafish ECGs to facilitate its routine application in preclinical investigation of QTc-prolonging drugs with reliable estimation of NOAEL. Our study is of relevance to the development and use of alternate animal models in drug discovery.

Elevated electrochemical impedance in the endoluminal regions with high shear stress: implication for assessing lipid-rich atherosclerotic lesions

BACKGROUND

Identifying metabolically active atherosclerotic lesions remains an unmet clinical challenge during coronary intervention. Electrochemical impedance (EIS) increased in response to oxidized low density lipoprotein (oxLDL)-laden lesions. We hereby assessed whether integrating EIS with intravascular ultrasound (IVUS) and shear stress (ISS) provided a new strategy to assess oxLDL-laden lesions in the fat-fed New Zealand White (NZW) rabbits.

METHODS AND RESULTS

A micro-heat transfer sensor was deployed to acquire the ISS profiles at baseline and post high-fat diet (HD) in the NZW rabbits (n=8). After 9 weeks of HD, serum oxLDL levels (mg/dL) increased by 140 fold, accompanied by a 1.5-fold increase in kinematic viscosity (cP) in the HD group. Time-averaged ISS (ISSave) in the thoracic aorta also increased in the HD group (baseline: 17.61±0.24 vs. 9 weeks: 25.22±0.95dyne/cm(2), n=4), but remained unchanged in the normal diet group (baseline: 22.85±0.53dyn/cm(2) vs. 9 weeks: 22.37±0.57dyne/cm(2), n=4). High-frequency intravascular ultrasound (IVUS) revealed atherosclerotic lesions in the regions with augmented ISSave, and concentric bipolar microelectrodes demonstrated elevated EIS signals, which were correlated with prominent anti-oxLDL immuno-staining (oxLDL-free regions: 497±55Ω, n=8 vs. oxLDL-rich lesions: 679±125Ω, n=12, P<0.05). The equivalent circuit model for tissue resistance between the lesion-free and ox-LDL-rich lesions further validated the experimental EIS signals.

CONCLUSIONS

By applying electrochemical impedance in conjunction with shear stress and high-frequency ultrasound sensors, we provided a new strategy to identify oxLDL-laden lesions. The study demonstrated the feasibility of integrating EIS, ISS, and IVUS for a catheter-based approach to assess mechanically unstable plaque.

Flexible microelectrode arrays to interface epicardial electrical signals with intracardial calcium transients in zebrafish hearts

The zebrafish (Danio rerio) is an emerging genetic model for regenerative medicine. In humans, myocardial infarction results in the irreversible loss of cardiomyocytes. However, zebrafish hearts fully regenerate after a 20% ventricular resection, without either scarring or arrhythmias. To study this cardiac regeneration, we developed implantable flexible multi-microelectrode membrane arrays that measure the epicardial electrocardiogram signals of zebrafish in real-time. The microelectrode electrical signals allowed for a high level of both temporal and spatial resolution (~20 μm), and the signal to noise ratio of the epicardial ECG was comparable to that of surface electrode ECG (7.1 dB vs. 7.4 dB, respectively). Processing and analysis of the signals from the microelectrode array demonstrated distinct ECG signals: namely, atrial conduction (P waves), ventricular contraction (QRS), and ventricular repolarization (QT interval). The electrical signals were in synchrony with optically measured Calcium concentration gradients in terms of d[Ca²⁺]/dt at both whole heart and tissue levels. These microelectrodes therefore provide a real-time analytical tool for monitoring conduction phenotypes of small vertebral animals with a high temporal and spatial resolution.

Imaging escape and avoidance behavior in zebrafish larvae

This review provides an overview of the assays that are used for measuring escape and avoidance behavior in zebrafish, with a specific focus on zebrafish larvae during the first week of development. Zebrafish larvae display a startle response when exposed to tactile, acoustic, or visual stimuli and will avoid dark areas, moving objects, conspecifics, and open spaces. Emotional states such as fear and anxiety might be induced when larvae are exposed to stimuli that they would normally escape from or avoid. Although these emotional states probably differ between species and change during development, much can be learned about human fear and anxiety using zebrafish as a model system. The molecular mechanisms of fear and anxiety are highly conserved in vertebrates and are present during early zebrafish development. Larvae during the first week of development display elevated cortisol levels in response to stress and are sensitive to the same anxiolytics that are used for the management of anxiety in humans. Zebrafish larvae are well suited for high-throughput analyses of behavior, and automated systems have been developed for imaging and analyzing the behavior of zebrafish larvae in multiwell plates. These high-throughput analyses will not only provide a wealth of information on the genes and environmental factors that influence escape and avoidance behaviors and the emotional states that might accompany them but will also facilitate the discovery of novel pharmaceuticals that could be used in the management of anxiety disorders in humans.

The zebrafish heart regenerates after cryoinjury-induced myocardial infarction

Background

In humans, myocardial infarction is characterized by irreversible loss of heart tissue, which becomes replaced with a fibrous scar. By contrast, teleost fish and urodele amphibians are capable of heart regeneration after a partial amputation. However, due to the lack of a suitable infarct model, it is not known how these animals respond to myocardial infarction.

Results

Here, we have established a heart infarct model in zebrafish using cryoinjury. In contrast to the common method of partial resection, cryoinjury results in massive cell death within 20% of the ventricular wall, similar to that observed in mammalian infarcts. As in mammals, the initial stages of the injury response include thrombosis, accumulation of fibroblasts and collagen deposition. However, at later stages, cardiac cells can enter the cell cycle and invade the infarct area in zebrafish. In the subsequent two months, fibrotic scar tissue is progressively eliminated by cell apoptosis and becomes replaced with a new myocardium, resulting in scarless regeneration. We show that tissue remodeling at the myocardial-infarct border zone is associated with accumulation of Vimentin-positive fibroblasts and with expression of an extracellular matrix protein Tenascin-C. Electrocardiogram analysis demonstrated that the reconstitution of the cardiac muscle leads to the restoration of the heart function.

Conclusions

We developed a new cryoinjury model to induce myocardial infarction in zebrafish. Although the initial stages following cryoinjury resemble typical healing in mammals, the zebrafish heart is capable of structural and functional regeneration. Understanding the key healing processes after myocardial infarction in zebrafish may result in identification of the barriers to efficient cardiac regeneration in mammals.

MEMS thermal sensors to detect changes in heat transfer in the pre-atherosclerotic regions of fat-fed New Zealand white rabbits

Real-time detection of pre-atherosclerotic regions remains an unmet clinical challenge. We previously demonstrated the application of micro-electro-mechanical systems (MEMS) to detect changes in convective heat transfer in terms of sensor output voltages in the zone of flow reversal in an in vitro stenotic model. We hereby demonstrated changes in sensor output voltages in the pre-atherosclerotic regions in the New Zealand White rabbits fed on hypercholesterolemic diet (HD). After 8 weeks, we observed that mean output voltages (V(ave)) were similar in the distal aortic arch, thoracic, and abdominal aortas in the normal standard diet (ND) group, consistent with an absence of atherosclerosis. In HD group, V(ave) increased in the distal aortic arch (HD: V(ave) = 1.05 ± 0.04 V; ND: V(ave) = 0.12 ± 0.01 V, n = 3, p < 0.05) and in the thoracic aortas (HD: V(ave) = 0.72 ± 0.06 V; ND: V(ave) = 0.13 ± 0.024 V, n = 3, p < 0.05), consistent with the histological presence of pre-atherosclerosis. Despite HD diet, V (ave) magnitudes were similar to ND group in the abdominal aortas (HD: V(ave) = 0.14 ± 0.003 V; ND: V(ave) = 0.14 ± 0.004 V, n = 3), corroborating histological absence of pre-atherosclerosis. Hence, MEMS thermal sensors provide a new approach to detect changes in convective heat transfer in the pre-atherosclerotic regions.

Combined use of MS-222 (tricaine) and isoflurane extends anesthesia time and minimizes cardiac rhythm side effects in adult zebrafish

As an important vertebrate model organism, zebrafish are typically studied at the embryonic stage to take advantage of their properties of transparency and rapid development. However, more and more studies require assays to be done on adults. Consequently, a good anesthetic is needed to sedate and immobilize the adult zebrafish during experimental manipulation. To date, MS-222 (tricaine methanesulfonate) is the only Food and Drug Administration approved anesthetic for aquaculture and is widely used by the zebrafish research community. Nevertheless, in adult zebrafish, MS-222 reduces heart rate and causes high mortality under long-term sedation. Consequently, adult zebrafish have limited research applications. In this study, we present a new anesthetic formula for the adult zebrafish that results in minimal side effects on its physiology under prolonged sedation. The combined use of MS-222 with isoflurane effectively extended the time of anesthesia, and the zebrafish recovered faster than when anesthetized with the traditional MS-222. Moreover, MS-222 + isoflurane did not cause reduction of heart rates, which enabled long-term electrocardiogram recording and microscopic observation on the adult zebrafish. Taken together, the new MS-222 + isoflurane formula will facilitate general applications of adult zebrafish in time-consuming experiments with minimal side effects on the model organism's overall physiology.

Evolving cardiac conduction phenotypes in developing zebrafish larvae: implications to drug sensitivity

Cardiac arrhythmias include problems with impulse formation and/or conduction abnormalities. Zebrafish (Danio rerio) is an emerging model system for studying the cardiac conduction system. However, real-time recording of the electrocardiogram remains a challenge. In the present study, we assessed the feasibility of recording electrical cardiogram (ECG) signals from the zebrafish larvae using the micropipette electrodes, and demonstrated the dynamic changes in ECG signals and their sensitivity to Amiodarone during the developmental stages. We observed that ECG signals revealed P waves and QRS complexes at 7 days postfertilization (dpf). T waves started to develop at 14 dpf. Distinct P waves, QRS complexes, and T waves were similar to those of adult zebrafish at 35 dpf, accompanied by a statistically significant decrease in QRS intervals (from 256 ± 16 ms at 7 dpf to 54 ± 6 ms, p < 0.01, n = 5). In response to Amiodarone, ECG signals showed QRS prolongation from 7 to 35 dpf (p < 0.05, n = 5). Hence, micropipette electrodes can be applied to detect evolving ECG signals from the developing zebrafish larvae, thus providing a noninvasive and nonparalyzing approach to investigate cardiac conduction phenotypes in response to genetic, epigenetic, or pharmacologic perturbation.

Real-time assessment of flow reversal in an eccentric arterial stenotic model

Plaque rupture is the leading cause of acute coronary syndromes and stroke. Plaque formation, otherwise known as stenosis, preferentially occurs in the regions of arterial bifurcation or curvatures. To date, real-time assessment of stenosis-induced flow reversal remains a clinical challenge. By interfacing microelectromechanical system (MEMS) thermal sensors with the high frequency pulsed wave (PW) Doppler ultrasound, we proposed to assess flow reversal in the presence of an eccentric stenosis. We developed a 3-D stenotic model (inner diameter of 6mm, an eccentric stenosis with a height of 2.75 mm, and width of 21 mm) simulating a superficial arterial vessel. We demonstrated that heat transfer from the sensing element (2 x 80 μm²) to the flow field peaked as a function of flow rates at the throat of the stenosis along the center/midline of arterial model, and dropped downstream from the stenosis, where flow reversal was detected by the high frequency ultrasound device at 45 MHz. Computational fluid dynamics (CFD) codes are in agreement with the ultrasound-acquired flow profiles upstream, downstream, and at the throat of the stenosis. Hence, we characterized regions of eccentric stenosis in terms of changes in heat transfer along the midline of vessel and identified points of flow reversal with high spatial and temporal resolution.

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.