Recent progress in the use of zebrafish for novel cardiac drug discovery

Comparison of cardiotoxicity induced by alectinib, apatinib, lenvatinib and anlotinib in zebrafish embryos

Four tyrosine kinase inhibitors, alectinib, apatinib, lenvatinib and anlotinib, have been shown to be effective in the treatment of clinical tumors, but their cardiac risks have also raised concerns. In this study, zebrafish embryos at 6 h post fertilization (hpf) were exposed to the four drugs at concentrations of 0.05-0.2 mg/L until 72 hpf, and then the development of these embryos was quantified, including heart rate, body length, yolk sac area, pericardial area, distance between venous sinus and balloon arteriosus (SV-BA), separation of cardiac myocytes and endocardium, gene expression, vascular development and oxidative stress. At the same exposure concentrations, alectinib and apatinib had little effect on the cardiac development of zebrafish embryos, while lenvatinib and anlotinib could induce significant cardiotoxicity and developmental toxicity, including shortened of body length, delayed absorption of yolk sac, pericardial edema, prolonged SV-BA distance, separation of cardiomyocytes and endocardial cells, and downregulation of key genes for heart development. Heart rate decreased in all four drug treatment groups. In terms of vascular development, alectinib and apatinib did not inhibit the growth of embryonic intersegmental vessels (ISVs) and retinal vessels, while lenvatinib and anlotinib caused serious vascular toxicity, and the inhibition of anlotinib in vascular development was more obvious. Besides, the level of reactive oxygen species (ROS) in the lenvatinib and anlotinib treatment groups was significantly increased. Our results provide reference for comparing the cardiotoxicity of the four drugs.

Zebrafish as model system for the biological characterization of CK1 inhibitors

Introduction: The CK1 family is involved in a variety of physiological processes by regulating different signaling pathways, including the Wnt/β-catenin, the Hedgehog and the p53 signaling pathways. Mutations or dysregulation of kinases in general and of CK1 in particular are known to promote the development of cancer, neurodegenerative diseases and inflammation. There is increasing evidence that CK1 isoform specific small molecule inhibitors, including CK1δ- and CK1ε-specific inhibitors of Wnt production (IWP)-based small molecules with structural similarity to benzimidazole compounds, have promising therapeutic potential. Methods: In this study, we investigated the suitability of the zebrafish model system for the evaluation of such CK1 inhibitors. To this end, the kinetic parameters of human CK1 isoforms were compared with those of zebrafish orthologues. Furthermore, the effects of selective CK1δ inhibition during zebrafish embryonic development were analyzed in vivo. Results: The results revealed that zebrafish CK1δA and CK1δB were inhibited as effectively as human CK1δ by compounds G2-2 with IC50 values of 345 and 270 nM for CK1δA and CK1δB versus 503 nM for human CK1δ and G2-3 exhibiting IC50 values of 514 and 561 nM for zebrafish CK1δA and B, and 562 nM for human CK1δ. Furthermore, the effects of selective CK1δ inhibition on zebrafish embryonic development in vivo revealed phenotypic abnormalities indicative of downregulation of CK1δ. Treatment of zebrafish embryos with selected inhibitors resulted in marked phenotypic changes including blood stasis, heart failure, and tail malformations. Conclusion: The results suggest that the zebrafish is a suitable in vivo assay model system for initial studies of the biological relevance of CK1δ inhibition.

Embryonic exposure to acetyl-L-carnitine protects against valproic acid-induced cardiac malformation in zebrafish model

Worldwide, estimated counts of about 7.9 million children are born with serious birth defects. In addition to genetic factors, prenatal exposure to drugs and environmental toxicants represents a major contributing factor to congenital malformations. In earlier investigation, we explored cardiac malformation caused by valproic acid (VPA) during early developing stages of zebrafish. Since heart depends on mitochondrial fatty acid oxidative metabolism for energy demands in which carnitine shuttle has a major role, the present study aimed to investigate the effect of acetyl-L-carnitine (AC) against VPA-induced cardiac malformation in developing zebrafish. Initially, AC was subjected to toxicological evaluation, and two micromolar concentrations (25 µM and 50 µM) were selected for evaluation. A sub-lethal concentration of VPA (50 µM) was selected to induce cardiac malformation. The embryos were grouped and the drug exposures were made at 2.5 h post-fertilization (hpf). The cardiac development and functioning was monitored. A progressive decline in cardiac functioning was noted in group exposed to VPA 50 µM. At 96 hpf and 120 hpf, the morphology of heart was severely affected with the chambers which became elongated and string-like accompanied by histological changes. Acridine orange staining showed accumulation of apoptotic cells. Group exposed to VPA 50 µM with AC 50 µM showed a significant reduction in pericardial sac edema with morphological, functional and histological recovery in developing heart. Moreover, reduced number of apoptotic cells was noted. The improvement with AC might be due to restoration of carnitine homeostasis for cardiac energy metabolism in developing heart.

Microengineered platforms for characterizing the contractile function of in vitro cardiac models

Emerging heart-on-a-chip platforms are promising approaches to establish cardiac cell/tissue models in vitro for research on cardiac physiology, disease modeling and drug cardiotoxicity as well as for therapeutic discovery. Challenges still exist in obtaining the complete capability of in situ sensing to fully evaluate the complex functional properties of cardiac cell/tissue models. Changes to contractile strength (contractility) and beating regularity (rhythm) are particularly important to generate accurate, predictive models. Developing new platforms and technologies to assess the contractile functions of in vitro cardiac models is essential to provide information on cell/tissue physiologies, drug-induced inotropic responses, and the mechanisms of cardiac diseases. In this review, we discuss recent advances in biosensing platforms for the measurement of contractile functions of in vitro cardiac models, including single cardiomyocytes, 2D monolayers of cardiomyocytes, and 3D cardiac tissues. The characteristics and performance of current platforms are reviewed in terms of sensing principles, measured parameters, performance, cell sources, cell/tissue model configurations, advantages, and limitations. In addition, we highlight applications of these platforms and relevant discoveries in fundamental investigations, drug testing, and disease modeling. Furthermore, challenges and future outlooks of heart-on-a-chip platforms for in vitro measurement of cardiac functional properties are discussed.

Progress in using zebrafish as a toxicological model for traditional Chinese medicine

ETHNOPHARMACOLOGICAL RELEVANCE

Traditional Chinese medicine (TCM) has been applied for more than 2000 years. However, modern basic research on the safety of TCMs is limited. Establishing safety evaluation technology in line with the characteristics of TCM and conducting large-scale basic toxicity research are keys to comprehensively understand the toxicity of TCMs. In recent years, zebrafish has been used as a model organism for toxicity assessment and is increasingly utilized for toxicity research of TCMs. Yet, a comprehensive review in using zebrafish as a toxicological model for TCMs is lacked.

AIM OF THE STUDY

We aim to summarize the progress and limitation in toxicity evaluation of TCMs using zebrafish and put forward the future research ideas.

MATERIALS AND METHODS

The scientific databases, including Springer, Science Direct, Wiley, Pubmed and China Knowledge Resource Integrated (CNKI) were searched using the key words of zebrafish, toxicology, traditional Chinese medicine, acute toxicity, liver injury, cardiotoxicity, kidney toxicity, developmental toxicity, neurotoxicity, gastrointestinal irritation, immunotoxicity, ototoxicity, and osteotoxicity.

RESULTS

Zebrafish assays are low experimental cost and short cycle, easily achieving high-throughput toxicity screening, and exemption from ethical legislation up to 5 dpf. It has been widely used to evaluate the acute toxicity, liver toxicity, cardiotoxicity, nephrotoxicity, developmental toxicity, neurotoxicity, gastrointestinal irritation, immunotoxicity, and ototoxicity caused by TCMs, although some physiological difference limited its application.

CONCLUSIONS

Zebrafish is a powerful model for TCMs toxicity evaluation, but it is not flawless. The toxicity testing criterion and high throughput assays are urgent to be established. This review provides references for future studies.

Experimental models of Barth syndrome

Mutation of the gene Tafazzin (TAZ) causes Barth syndrome, an X-linked disorder characterized by cardiomyopathy, skeletal muscle weakness, and neutropenia. TAZ is an acyltransferase that catalyzes the remodeling of cardiolipin, the signature phospholipid of the inner mitochondrial membrane. Here, we review the major model systems that have been established to study the role of cardiolipin remodeling in mitochondrial function and the pathogenesis of Barth syndrome. We summarize key features of each model and provide examples of how each has contributed to advance our understanding of TAZ function and Barth syndrome pathophysiology.

Zebrafish Heart Failure Models

Heart failure causes significant morbidity and mortality worldwide. The understanding of heart failure pathomechanisms and options for treatment remain incomplete. Zebrafish has proven useful for modeling human heart diseases due to similarity of zebrafish and mammalian hearts, fast easily tractable development, and readily available genetic methods. Embryonic cardiac development is rapid and cardiac function is easy to observe and quantify. Reverse genetics, by using morpholinos and CRISPR-Cas9 to modulate gene function, make zebrafish a primary animal model for in vivo studies of candidate genes. Zebrafish are able to effectively regenerate their hearts following injury. However, less attention has been given to using zebrafish models to increase understanding of heart failure and cardiac remodeling, including cardiac hypertrophy and hyperplasia. Here we discuss using zebrafish to study heart failure and cardiac remodeling, and review zebrafish genetic, drug-induced and other heart failure models, discussing the advantages and weaknesses of using zebrafish to model human heart disease. Using zebrafish models will lead to insights on the pathomechanisms of heart failure, with the aim to ultimately provide novel therapies for the prevention and treatment of heart failure.

The Diversity of Muscles and Their Regenerative Potential across Animals

Cells with contractile functions are present in almost all metazoans, and so are the related processes of muscle homeostasis and regeneration. Regeneration itself is a complex process unevenly spread across metazoans that ranges from full-body regeneration to partial reconstruction of damaged organs or body tissues, including muscles. The cellular and molecular mechanisms involved in regenerative processes can be homologous, co-opted, and/or evolved independently. By comparing the mechanisms of muscle homeostasis and regeneration throughout the diversity of animal body-plans and life cycles, it is possible to identify conserved and divergent cellular and molecular mechanisms underlying muscle plasticity. In this review we aim at providing an overview of muscle regeneration studies in metazoans, highlighting the major regenerative strategies and molecular pathways involved. By gathering these findings, we wish to advocate a comparative and evolutionary approach to prompt a wider use of “non-canonical” animal models for molecular and even pharmacological studies in the field of muscle regeneration.

Muscarinic receptors promote pacemaker fate at the expense of secondary conduction system tissue in zebrafish

Deterioration or inborn malformations of the cardiac conduction system (CCS) interfere with proper impulse propagation in the heart and may lead to sudden cardiac death or heart failure. Patients afflicted with arrhythmia depend on antiarrhythmic medication or invasive therapy, such as pacemaker implantation. An ideal way to treat these patients would be CCS tissue restoration. This, however, requires precise knowledge regarding the molecular mechanisms underlying CCS development. Here, we aimed to identify regulators of CCS development. We performed a compound screen in zebrafish embryos and identified tolterodine, a muscarinic receptor antagonist, as a modifier of CCS development. Tolterodine provoked a lower heart rate, pericardiac edema, and arrhythmia. Blockade of muscarinic M3, but not M2, receptors induced transcriptional changes leading to amplification of sinoatrial cells and loss of atrioventricular identity. Transcriptome data from an engineered human heart muscle model provided additional evidence for the contribution of muscarinic M3 receptors during cardiac progenitor specification and differentiation. Taken together, we found that muscarinic M3 receptors control the CCS already before the heart becomes innervated. Our data indicate that muscarinic receptors maintain a delicate balance between the developing sinoatrial node and the atrioventricular canal, which is probably required to prevent the development of arrhythmia.

Streamlining drug discovery assays for cardiovascular disease using zebrafish

Introduction: In the last decade, our armamentarium of cardiovascular drug therapy has expanded significantly. Using innovative functional genomics strategies such as genome editing by CRISPR/Cas9 as well as high-throughput assays to identify bioactive small chemical compounds has significantly facilitated elaboration of the underlying pathomechanism in various cardiovascular diseases. However, despite scientific progress approvals for cardiovascular drugs has stagnated significantly compared to other fields of drug discovery and therapy during the past years.Areas covered: In this review, the authors discuss the aspects and pitfalls during the early phase of cardiovascular drug discovery and describe the advantages of zebrafish as an in vivo organism to model human cardiovascular diseases (CVD) as well as an in vivo platform for high-throughput chemical compound screening. They also highlight the emerging, promising techniques of automated read-out systems during high-throughput screening (HTS) for the evaluation of important cardiac functional parameters in zebrafish with the potential to streamline CVD drug discovery.Expert opinion: The successful identification of novel drugs to treat CVD is a major challenge in modern biomedical and clinical research. In this context, the definition of the etiologic fundamentals of human cardiovascular diseases is the prerequisite for an efficient and straightforward drug discovery.

Testing the Translational Power of the Zebrafish: An Interspecies Analysis of Responses to Cardiovascular Drugs

The zebrafish is rapidly emerging as a promising alternative in vivo model for the detection of drug-induced cardiovascular effects. Despite its increasing popularity, the ability of this model to inform the drug development process is often limited by the uncertainties around the quantitative relevance of zebrafish responses compared with nonclinical mammalian species and ultimately humans. In this test of concept study, we provide a comparative quantitative analysis of the in vivo cardiovascular responses of zebrafish, rat, dog, and human to three model compounds (propranolol, losartan, and captopril), which act as modulators of two key systems (beta-adrenergic and renin–angiotensin systems) involved in the regulation of cardiovascular functions. We used in vivo imaging techniques to generate novel experimental data of drug-mediated cardiovascular effects in zebrafish larvae. These data were combined with a database of interspecies mammalian responses (i.e., heart rate, blood flow, vessel diameter, and stroke volume) extracted from the literature to perform a meta-analysis of effect size and direction across multiple species. In spite of the high heterogeneity of study design parameters, our analysis highlighted that zebrafish and human responses were largely comparable in >80% of drug/endpoint combinations. However, it also revealed a high intraspecies variability, which, in some cases, prevented a conclusive interpretation of the drug-induced effect. Despite the shortcomings of our study, the meta-analysis approach, combined with a suitable data visualization strategy, enabled us to observe patterns of response that would likely remain undetected with more traditional methods of qualitative comparative analysis. We propose that expanding this approach to larger datasets encompassing multiple drugs and modes of action would enable a rigorous and systematic assessment of the applicability domain of the zebrafish from both a mechanistic and phenotypic standpoint. This will increase the confidence in its application for the early detection of adverse drug reactions in any major organ system.

Genetics of Cardiovascular Disease: Fishing for Causality

Cardiovascular disease (CVD) is still the leading cause of death in all western world countries and genetic predisposition in combination with traditional risk factors frequently mediates their manifestation. Genome-wide association (GWA) studies revealed numerous potentially disease modifying genetic loci often including several SNPs and associated genes. However, pure genetic association does not prove direct or indirect relevance of the modifier region on pathogenesis, nor does it define within the associated region the exact genetic driver of the disease. Therefore, the relevance of the identified genetic disease associations needs to be confirmed either in monogenic traits or in experimental in vivo model system by functional genomic studies. In this review, we focus on the use of functional genomic approaches such as gene knock-down or CRISPR/Cas9-mediated genome editing in the zebrafish model to validate disease-associated genomic loci and to identify novel cardiovascular disease genes. We summarize the benefits of the zebrafish for cardiovascular research and highlight examples demonstrating the successful combination of GWA studies and functional genomics in zebrafish to broaden our knowledge on the genetic and molecular underpinnings of cardiovascular diseases.

New perspectives: systems medicine in cardiovascular disease

Background

Cardiovascular diseases (CVD) represent one of the most important causes of morbidity and mortality worldwide. Innovative approaches to increase the understanding of the underpinnings of CVD promise to enhance CVD risk assessment and might pave the way to tailored therapies. Within the last years, systems medicine has emerged as a novel tool to study the genetic, molecular and physiological interactions.

Conclusion

In this review, we provide an overview of the current molecular-experimental, epidemiological and bioinformatical tools applied in systems medicine in the cardiovascular field. We will discuss the status and challenges in implementing interdisciplinary systems medicine approaches in CVD.

Development of a high-throughput in vivo screening platform for particulate matter exposures

Particulate matter (PM) exposure is a public health burden with poorly understood health effect mechanisms and lacking an efficient model to compare the vast diversity of PM exposures. Zebrafish (Danio rerio) are amenable to high-throughput screening (HTS), but few studies have investigated PM toxicity in zebrafish, despite the multitude of advantages. To develop standardized exposure procedures, the urban PM standard reference material (SRM) 1649b was used to systematically determine sample preparation methods, design experimental controls, determine concentration ranges and evaluation procedures. Embryos (n = 32/treatment) were dechorionated and placed into 96-well plates containing SRM1649b (0-200 μg/mL) at 6 h post fertilization (hpf). Developmental toxicity was assessed at 24 and 120 hpf by evaluating morphological changes, embryonic/larval photomotor behavior, and mortality. Differences from blank medium and particle controls were observed for all biological responses measured. Differences due to SRM1649b concentration and preparation method were also observed. Exposure to SRM1649b from DMSO extraction was associated with changes in morphology and mortality and hypoactivity in photomotor responses compared to the DMSO control for the whole particle suspension (76, 68%) and soluble fraction (59, 54%) during the embryonic and larval stages, respectively. Changes in behavioral responses were not observed following exposure to the insoluble fraction of SRM1649b from DMSO extraction. The toxicity bias from PM preparation provided further impetus to select a single HTS exposure method. Based on the biological activity results, the soluble fraction of SRM1649b from DMSO extraction was selected and shown to have concentration dependent cyp1a/GFP expression. This rapid, sensitive and consistently scalable model is a potentially cost-effective vertebrate approach to study the toxicology of PM from diverse locations, and provides a path to identifying the toxic material(s) in these samples, and discover the mechanisms of toxicity.

Zebrafish as a Model for Drug Screening in Genetic Kidney Diseases

Genetic disorders account for a wide range of renal diseases emerging during childhood and adolescence. Due to the utilization of modern biochemical and biomedical techniques, the number of identified disease-associated genes is increasing rapidly. Modeling of congenital human disease in animals is key to our understanding of the biological mechanism underlying pathological processes and thus developing novel potential treatment options. The zebrafish (Danio rerio) has been established as a versatile small vertebrate organism that is widely used for studying human inherited diseases. Genetic accessibility in combination with elegant experimental methods in zebrafish permit modeling of human genetic diseases and dissecting the perturbation of underlying cellular networks and physiological processes. Beyond its utility for genetic analysis and pathophysiological and mechanistic studies, zebrafish embryos, and larvae are amenable for phenotypic screening approaches employing high-content and high-throughput experiments using automated microscopy. This includes large-scale chemical screening experiments using genetic models for searching for disease-modulating compounds. Phenotype-based approaches of drug discovery have been successfully performed in diverse zebrafish-based screening applications with various phenotypic readouts. As a result, these can lead to the identification of candidate substances that are further examined in preclinical and clinical trials. In this review, we discuss zebrafish models for inherited kidney disease as well as requirements and considerations for the technical realization of drug screening experiments in zebrafish.

Modeling Alzheimer's and Other Age Related Human Diseases in Embryonic Systems

Modeling human disease in animals is an important strategy to discover potential methods of intervention. We suggest that there is much to be gained by employing a multi-model approach that takes advantage of different animal systems used in the laboratory simultaneously. We use the example of modeling Alzheimer's disease in Drosophila melanogaster, Caenorhabditis elegans, and Danio rerio to illustrate how such an approach can be employed to investigate the pathophysiology of the disease.

Automatic determination of heart rates from microscopy videos of early life stages of fish

Toxic effects of organic hydrophobic contaminants include impacts on fish heart rate (HR) and cardiac functioning. Thus, in ecotoxicology as well as aquaculture and even medicine, fish heart functioning plays an important role in application areas. The aim of this study was to assemble a pipeline of image processing and statistical techniques to extract HR information from microscopy videos of the embryo and larval stages of three species of fish (Atlantic cod, haddock, and Atlantic bluefin tuna). The method enables automatic processing for a large number of individuals, saving a significant amount of time compared with manual processing, while simultaneously eliminating the type of errors such a manual process might incur.

On the Evolution of the Cardiac Pacemaker

The rhythmic contraction of the heart is initiated and controlled by an intrinsic pacemaker system. Cardiac contractions commence at very early embryonic stages and coordination remains crucial for survival. The underlying molecular mechanisms of pacemaker cell development and function are still not fully understood. Heart form and function show high evolutionary conservation. Even in simple contractile cardiac tubes in primitive invertebrates, cardiac function is controlled by intrinsic, autonomous pacemaker cells. Understanding the evolutionary origin and development of cardiac pacemaker cells will help us outline the important pathways and factors involved. Key patterning factors, such as the homeodomain transcription factors Nkx2.5 and Shox2, and the LIM-homeodomain transcription factor Islet-1, components of the T-box (Tbx), and bone morphogenic protein (Bmp) families are well conserved. Here we compare the dominant pacemaking systems in various organisms with respect to the underlying molecular regulation. Comparative analysis of the pathways involved in patterning the pacemaker domain in an evolutionary context might help us outline a common fundamental pacemaker cell gene programme. Special focus is given to pacemaker development in zebrafish, an extensively used model for vertebrate development. Finally, we conclude with a summary of highly conserved key factors in pacemaker cell development and function.

Amplified pathogenic actions of angiotensin II in cysteine-rich LIM-only protein 4-negative mouse hearts

LIM domain proteins have been identified as essential modulators of cardiac biology and pathology; however, it is unclear which role the cysteine-rich LIM-only protein (CRP)4 plays in these processes. In studying CRP4 mutant mice, we found that their hearts developed normally, but lack of CRP4 exaggerated multiple parameters of the cardiac stress response to the neurohormone angiotensin II (Ang II). Aiming to dissect the molecular details, we found a link between CRP4 and the cardioprotective cGMP pathway, as well as a multiprotein complex comprising well-known hypertrophy-associated factors. Significant enrichment of the cysteine-rich intestinal protein (CRIP)1 in murine hearts lacking CRP4, as well as severe cardiac defects and premature death of CRIP1 and CRP4 morphant zebrafish embryos, further support the notion that depleting CRP4 is incompatible with a proper cardiac development and function. Together, amplified Ang II signaling identified CRP4 as a novel antiremodeling factor regulated, at least to some extent, by cardiac cGMP.-Straubinger, J., Boldt, K., Kuret, A., Deng, L., Krattenmacher, D., Bork, N., Desch, M., Feil, R., Feil, S., Nemer, M., Ueffing, M., Ruth, P., Just, S., Lukowski, R. Amplified pathogenic actions of angiotensin II in cysteine-rich LIM-only protein 4 negative mouse hearts.

Using CRISPR/Cas9-Mediated GLA Gene Knockout as an In Vitro Drug Screening Model for Fabry Disease

The CRISPR/Cas9 Genome-editing system has revealed promising potential for generating gene mutation, deletion, and correction in human cells. Application of this powerful tool in Fabry disease (FD), however, still needs to be explored. Enzyme replacement therapy (ERT), a regular administration of recombinant human α Gal A (rhα-GLA), is a currently available and effective treatment to clear the accumulated Gb3 in FD patients. However, the short half-life of rhα-GLA in human body limits its application. Moreover, lack of an appropriate in vitro disease model restricted the high-throughput screening of drugs for improving ERT efficacy. Therefore, it is worth establishing a large-expanded in vitro FD model for screening potential candidates, which can enhance and prolong ERT potency. Using CRISPR/Cas9-mediated gene knockout of GLA in HEK-293T cells, we generated GLA-null cells to investigate rhα-GLA cellular pharmacokinetics. The half-life of administrated rhα-GLA was around 24 h in GLA-null cells; co-administration of proteasome inhibitor MG132 and rhα-GLA significantly restored the GLA enzyme activity by two-fold compared with rhα-GLA alone. Furthermore, co-treatment of rhα-GLA/MG132 in patient-derived fibroblasts increased Gb3 clearance by 30%, compared with rhα-GLA treatment alone. Collectively, the CRISPR/Cas9-mediated GLA-knockout HEK-293T cells provide an in vitro FD model for evaluating the intracellular pharmacokinetics of the rhα-GLA as well as for screening candidates to prolong rhα-GLA potency. Using this model, we demonstrated that MG132 prolongs rhα-GLA half-life and enhanced Gb3 clearance, shedding light on the direction of enhancing ERT efficacy in FD treatment.

Advances in the Study of Heart Development and Disease Using Zebrafish

Animal models of cardiovascular disease are key players in the translational medicine pipeline used to define the conserved genetic and molecular basis of disease. Congenital heart diseases (CHDs) are the most common type of human birth defect and feature structural abnormalities that arise during cardiac development and maturation. The zebrafish, Danio rerio, is a valuable vertebrate model organism, offering advantages over traditional mammalian models. These advantages include the rapid, stereotyped and external development of transparent embryos produced in large numbers from inexpensively housed adults, vast capacity for genetic manipulation, and amenability to high-throughput screening. With the help of modern genetics and a sequenced genome, zebrafish have led to insights in cardiovascular diseases ranging from CHDs to arrhythmia and cardiomyopathy. Here, we discuss the utility of zebrafish as a model system and summarize zebrafish cardiac morphogenesis with emphasis on parallels to human heart diseases. Additionally, we discuss the specific tools and experimental platforms utilized in the zebrafish model including forward screens, functional characterization of candidate genes, and high throughput applications.

Atrogin-1 Deficiency Leads to Myopathy and Heart Failure in Zebrafish

Orchestrated protein synthesis and degradation is fundamental for proper cell function. In muscle, impairment of proteostasis often leads to severe cellular defects finally interfering with contractile function. Here, we analyze for the first time the role of Atrogin-1, a muscle-specific E3 ubiquitin ligase known to be involved in the regulation of protein degradation via the ubiquitin proteasome and the autophagy/lysosome systems, in the in vivo model system zebrafish (Danio rerio). We found that targeted inactivation of zebrafish Atrogin-1 leads to progressive impairment of heart and skeletal muscle function and disruption of muscle structure without affecting early cardiogenesis and skeletal muscle development. Autophagy is severely impaired in Atrogin-1-deficient zebrafish embryos resulting in the disturbance of the cytoarchitecture of cardiomyocytes and skeletal muscle cells. These observations are consistent with molecular and ultrastructural findings in an Atrogin-1 knockout mouse and demonstrate that the zebrafish is a suitable vertebrate model to study the molecular mechanisms of Atrogin-1-mediated autophagic muscle pathologies and to screen for novel therapeutically active substances in high-throughput in vivo small compound screens (SCS).

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.