Modeling Inherited Cardiomyopathies in Adult Zebrafish for Precision Medicine

Allelic heterogeneity of TTNtv dilated cardiomyopathy can be modeled in adult zebrafish

Allelic heterogeneity (AH) has been noted in truncational TTN-associated (TTNtv-associated) dilated cardiomyopathy (DCM); i.e., mutations affecting A-band-encoding exons are pathogenic, but those affecting Z-disc-encoding exons are likely benign. The lack of an in vivo animal model that recapitulates AH hinders the deciphering of the underlying mechanism. Here, we explored zebrafish as a candidate vertebrate model by phenotyping a collection of zebrafish ttntv alleles. We noted that cardiac function and sarcomere structure were more severely disrupted in ttntv-A than in ttntv-Z homozygous embryos. Consistently, cardiomyopathy-like phenotypes were present in ttntv-A but not ttntv-Z adult heterozygous mutants. The phenotypes observed in ttntv-A alleles were recapitulated in null mutants with the full titin-encoding sequences removed. Defective autophagic flux, largely due to impaired autophagosome-lysosome fusion, was also noted only in ttntv-A but not in ttntv-Z models. Moreover, we found that genetic manipulation of ulk1a restored autophagy flux and rescued cardiac dysfunction in ttntv-A animals. Together, our findings presented adult zebrafish as an in vivo animal model for studying AH in TTNtv DCM, demonstrated TTN loss of function is sufficient to trigger ttntv DCM in zebrafish, and uncovered ulk1a as a potential therapeutic target gene for TTNtv DCM.

V. S, Kaushik K, Sabharwal A, Lalwani MK, K. S, Singh N, Scaria V, Sivasubbu S. Forward genetic screen using a gene-breaking trap approach identifies a novel role of grin2bb-associated RNA transcript (grin2bbART) in zebrafish heart function

LncRNA-based control affects cardiac pathophysiologies like myocardial infarction, coronary artery disease, hypertrophy, and myotonic muscular dystrophy. This study used a gene-break transposon (GBT) to screen zebrafish (Danio rerio) for insertional mutagenesis. We identified three insertional mutants where the GBT captured a cardiac gene. One of the adult viable GBT mutants had bradycardia (heart arrhythmia) and enlarged cardiac chambers or hypertrophy; we named it "bigheart." Bigheart mutant insertion maps to grin2bb or N-methyl D-aspartate receptor (NMDAR2B) gene intron 2 in reverse orientation. Rapid amplification of adjacent cDNA ends analysis suggested a new insertion site transcript in the intron 2 of grin2bb. Analysis of the RNA sequencing of wild-type zebrafish heart chambers revealed a possible new transcript at the insertion site. As this putative lncRNA transcript satisfies the canonical signatures, we called this transcript grin2bb associated RNA transcript (grin2bbART). Using in situ hybridization, we confirmed localized grin2bbART expression in the heart, central nervous system, and muscles in the developing embryos and wild-type adult zebrafish atrium and bulbus arteriosus. The bigheart mutant had reduced Grin2bbART expression. We showed that bigheart gene trap insertion excision reversed cardiac-specific arrhythmia and atrial hypertrophy and restored grin2bbART expression. Morpholino-mediated antisense downregulation of grin2bbART in wild-type zebrafish embryos mimicked bigheart mutants; this suggests grin2bbART is linked to bigheart. Cardiovascular tissues use Grin2bb as a calcium-permeable ion channel. Calcium imaging experiments performed on bigheart mutants indicated calcium mishandling in the heart. The bigheart cardiac transcriptome showed differential expression of calcium homeostasis, cardiac remodeling, and contraction genes. Western blot analysis highlighted Camk2d1 and Hdac1 overexpression. We propose that altered calcium activity due to disruption of grin2bbART, a putative lncRNA in bigheart, altered the Camk2d-Hdac pathway, causing heart arrhythmia and hypertrophy in zebrafish.

4D Light-sheet Imaging of Zebrafish Cardiac Contraction

Zebrafish is an intriguing model organism known for its remarkable cardiac regeneration capacity. Studying the contracting heart in vivo is essential for gaining insights into structural and functional changes in response to injuries. However, obtaining high-resolution and high-speed 4-dimensional (4D, 3D spatial + 1D temporal) images of the zebrafish heart to assess cardiac architecture and contractility remains challenging. In this context, an in-house light-sheet microscope (LSM) and customized computational analysis are used to overcome these technical limitations. This strategy, involving LSM system construction, retrospective synchronization, single cell tracking, and user-directed analysis, enables one to investigate the micro-structure and contractile function across the entire heart at the single-cell resolution in the transgenic Tg(myl7:nucGFP) zebrafish larvae. Additionally, we are able to further incorporate microinjection of small molecule compounds to induce cardiac injury in a precise and controlled manner. Overall, this framework allows one to track physiological and pathophysiological changes, as well as the regional mechanics at the single-cell level during cardiac morphogenesis and regeneration.

Transcriptome studies of inherited dilated cardiomyopathies

Dilated cardiomyopathy (DCM) is a group of heart muscle diseases that often lead to heart failure, with more than 50 causative genes have being linked to DCM. The heterogenous nature of the inherited DCMs suggest the need of precision medicine. Consistent with this emerging concept, transcriptome studies in human patients with DCM indicated distinct molecular signature for DCMs of different genetic etiology. To facilitate this line of research, we reviewed the status of transcriptome studies of inherited DCMs by focusing on three predominant DCM causative genes, TTN, LMNA, and BAG3. Besides studies in human patients, we summarized transcriptomic analysis of these inherited DCMs in a variety of model systems ranging from iPSCs to rodents and zebrafish. We concluded that the RNA-seq technology is a powerful genomic tool that has already led to the discovery of new modifying genes, signaling pathways, and related therapeutic avenues. We also pointed out that both temporal (different pathological stages) and spatial (different cell types) information need to be considered for future transcriptome studies. While an important bottle neck is the low throughput in experimentally testing differentially expressed genes, new technologies in efficient animal models such as zebrafish starts to be developed. It is anticipated that the RNA-seq technology will continue to uncover both unique and common pathological events, aiding the development of precision medicine for inherited DCMs.

4D Light-sheet imaging and interactive analysis of cardiac contractility in zebrafish larvae

Despite ongoing efforts in cardiovascular research, the acquisition of high-resolution and high-speed images for the purpose of assessing cardiac contraction remains challenging. Light-sheet fluorescence microscopy (LSFM) offers superior spatiotemporal resolution and minimal photodamage, providing an indispensable opportunity for the in vivo study of cardiac micro-structure and contractile function in zebrafish larvae. To track the myocardial architecture and contractility, we have developed an imaging strategy ranging from LSFM system construction, retrospective synchronization, single cell tracking, to user-directed virtual reality (VR) analysis. Our system enables the four-dimensional (4D) investigation of individual cardiomyocytes across the entire atrium and ventricle during multiple cardiac cycles in a zebrafish larva at the cellular resolution. To enhance the throughput of our model reconstruction and assessment, we have developed a parallel computing-assisted algorithm for 4D synchronization, resulting in a nearly tenfold enhancement of reconstruction efficiency. The machine learning-based nuclei segmentation and VR-based interaction further allow us to quantify cellular dynamics in the myocardium from end-systole to end-diastole. Collectively, our strategy facilitates noninvasive cardiac imaging and user-directed data interpretation with improved efficiency and accuracy, holding great promise to characterize functional changes and regional mechanics at the single cell level during cardiac development and regeneration.

Zebrafish as a Model of Cardiac Pathology and Toxicity: Spotlight on Uremic Toxins

Chronic kidney disease (CKD) is an increasing health care problem. About 10% of the general population is affected by CKD, representing the sixth cause of death in the world. Cardiovascular events are the main mortality cause in CKD, with a cardiovascular risk 10 times higher in these patients than the rate observed in healthy subjects. The gradual decline of the kidney leads to the accumulation of uremic solutes with a negative effect on every organ, especially on the cardiovascular system. Mammalian models, sharing structural and functional similarities with humans, have been widely used to study cardiovascular disease mechanisms and test new therapies, but many of them are rather expensive and difficult to manipulate. Over the last few decades, zebrafish has become a powerful non-mammalian model to study alterations associated with human disease. The high conservation of gene function, low cost, small size, rapid growth, and easiness of genetic manipulation are just some of the features of this experimental model. More specifically, embryonic cardiac development and physiological responses to exposure to numerous toxin substances are similar to those observed in mammals, making zebrafish an ideal model to study cardiac development, toxicity, and cardiovascular disease.

Zebrafish Models of Cardiac Disease: From Fortuitous Mutants to Precision Medicine

Heart disease is the leading cause of death worldwide. Despite decades of research, most heart pathologies have limited treatments, and often the only curative approach is heart transplantation. Thus, there is an urgent need to develop new therapeutic approaches for treating cardiac diseases. Animal models that reproduce the human pathophysiology are essential to uncovering the biology of diseases and discovering therapies. Traditionally, mammals have been used as models of cardiac disease, but the cost of generating and maintaining new models is exorbitant, and the studies have very low throughput. In the last decade, the zebrafish has emerged as a tractable model for cardiac diseases, owing to several characteristics that made this animal popular among developmental biologists. Zebrafish fertilization and development are external; embryos can be obtained in high numbers, are cheap and easy to maintain, and can be manipulated to create new genetic models. Moreover, zebrafish exhibit an exceptional ability to regenerate their heart after injury. This review summarizes 25 years of research using the zebrafish to study the heart, from the classical forward screenings to the contemporary methods to model mutations found in patients with cardiac disease. We discuss the advantages and limitations of this model organism and introduce the experimental approaches exploited in zebrafish, including forward and reverse genetics and chemical screenings. Last, we review the models used to induce cardiac injury and essential ideas derived from studying natural regeneration. Studies using zebrafish have the potential to accelerate the discovery of new strategies to treat cardiac diseases.

An Olive Oil Mill Wastewater Extract Improves Chemotherapeutic Activity Against Breast Cancer Cells While Protecting From Cardiotoxicity

Cardiovascular toxicity in cancer patients receiving chemotherapy remains one of the most undesirable side effects, limiting the choice of the most efficient therapeutic regimen, including combinations of different anticancer agents. Anthracyclines (doxorubicin) and antimetabolites (5-fluorouracil (5-FU), capecitabine) are among the most known agents used in breast cancer and other neoplasms and are associated with cardiotoxic effects. Extra-virgin olive oil (EVOO) is rich in polyphenols endowed with antioxidant cardioprotective activities. Olive mill wastewater (OMWW), a waste product generated by EVOO processing, has been reported to be enriched in polyphenols. In this study, we investigated the activities of polyphenol-rich extract from OMWW, A009, in cooperation with chemotherapy on two breast cancer cell lines, namely, BT459 and MDA-MB-231, in a cardio-oncology perspective. The effects of A009 on cardiac cells were also investigated with and without chemotherapeutic agents. Cell viability was determined on BT459 and MDA-MB-231 (i.e., breast cancer cells) and H9C2 (i.e., rat cardiomyocytes) cells, using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. A spheroids assay was used as a 3D in vitro model on BT459 and MDA-MB-231 cells. For in vivo studies, the murine sponge assay of angiogenesis was used as a model of breast cancer-associated vascularization. The embryo of Danio rerio (zebrafish) was used to detect the cardioprotective activities of the OMWW. We found that the A009 extract exhibited antiangiogenic activities induced by breast cancer cell supernatants and increased T-cell recruitment in vivo. The combination of the OMWW extracts with doxorubicin or 5-FU limited BT459 and MDA-MB-231 cell viability and the diameter of 3D spheroids, while mitigating their toxic effects on the rat H9C2 cardiomyocytes. Cardioprotective effects were observed by the combination of OMWW extracts with doxorubicin in zebrafish embryos. Finally, in human cardio myocytes, we observed 5-FU-induced upregulation of the inflammatory, senescence-associated cytokine IL6 and p16 genes, which expression was reduced by OMWW treatment. Our study demonstrates that the polyphenol-rich purified OMWW extract A009 combined with cancer chemotherapy could represent a potential candidate for cardiovascular protection in breast cancer patients, while increasing the effects of breast cancer chemotherapy.

Understanding the molecular basis of cardiomyopathy

Inherited cardiomyopathies are a major cause of mortality and morbidity worldwide and can be caused by mutations in a wide range of proteins located in different cellular compartments. The present review is based on Dr. Ju Chen's 2021 Robert M. Berne Distinguished Lectureship of the American Physiological Society Cardiovascular Section, in which he provided an overview of the current knowledge on the cardiomyopathy-associated proteins that have been studied in his laboratory. The review provides a general summary of the proteins in different compartments of cardiomyocytes associated with cardiomyopathies, with specific focus on the proteins that have been studied in Dr. Chen's laboratory.

A phenotype-based forward genetic screen identifies Dnajb6 as a sick sinus syndrome gene

Previously we showed the generation of a protein trap library made with the gene-break transposon (GBT) in zebrafish (Danio rerio) that could be used to facilitate novel functional genome annotation towards understanding molecular underpinnings of human diseases (Ichino et al, 2020). Here, we report a significant application of this library for discovering essential genes for heart rhythm disorders such as sick sinus syndrome (SSS). SSS is a group of heart rhythm disorders caused by malfunction of the sinus node, the heart's primary pacemaker. Partially owing to its aging-associated phenotypic manifestation and low expressivity, molecular mechanisms of SSS remain difficult to decipher. From 609 GBT lines screened, we generated a collection of 35 zebrafish insertional cardiac (ZIC) mutants in which each mutant traps a gene with cardiac expression. We further employed electrocardiographic measurements to screen these 35 ZIC lines and identified three GBT mutants with SSS-like phenotypes. More detailed functional studies on one of the arrhythmogenic mutants, GBT411, in both zebrafish and mouse models unveiled Dnajb6 as a novel SSS causative gene with a unique expression pattern within the subpopulation of sinus node pacemaker cells that partially overlaps with the expression of hyperpolarization activated cyclic nucleotide gated channel 4 (HCN4), supporting heterogeneity of the cardiac pacemaker cells.

A Swimming-based Assay to Determine the Exercise Capacity of Adult Zebrafish Cardiomyopathy Models

Exercise capacity, measured by treadmill in humans and other mammals, is an important diagnostic and prognostic index for patients with cardiomyopathy and heart failure. The adult zebrafish is increasingly used as a vertebrate model to study human cardiomyopathy due to its conserved cardiovascular physiology, convenience for genetic manipulation, and amenability to high-throughput genetic and compound screening. Owing to the small size of its body and heart, new phenotyping assays are needed to unveil phenotypic traits of cardiomyopathy in adult zebrafish. Here, we describe a swimming-based functional assay that measures exercise capacity in an adult zebrafish doxorubicin-induced cardiomyopathy model. This protocol can be applied to any adult zebrafish model of acquired or inherited cardiomyopathy and potentially to other cardiovascular diseases. Graphic abstract: Clinical relevance of the swimming-based phenotyping assay in adult zebrafish cardiomyopathy models.

Dilated cardiomyopathy: a new insight into the rare but common cause of heart failure

Heart failure is a global health burden responsible for high morbidity and mortality with a prevalence of greater than 60 million individuals worldwide. One of the major causes of heart failure is dilated cardiomyopathy (DCM), characterized by associated systolic dysfunction. During the last few decades, there have been remarkable advances in our understanding about the genetics of dilated cardiomyopathy. The genetic causes were initially thought to be associated with mutations in genes encoding proteins that are localized to cytoskeleton and sarcomere only; however, with the advancement in mechanistic understanding, the roles of ion channels, Z-disc, mitochondria, nuclear proteins, cardiac transcription factors (e.g., NKX-2.5, TBX20, GATA4), and the factors involved in calcium homeostasis have also been identified and found to be implicated in both familial and sporadic DCM cases. During past few years, next-generation sequencing (NGS) has been established as a diagnostic tool for genetic analysis and it has added significantly to the existing candidate gene list for DCM. The animal models have also provided novel insights to develop a better treatment strategy based on phenotype-genotype correlation, epigenetic and phenomic profiling. Most of the DCM biomarkers that are used in routine genetic and clinical testing are structural proteins, but during the last few years, the role of mi-RNA has also emerged as a biomarker due to their accessibility through noninvasive methods. Our increasing genetic knowledge can improve the clinical management of DCM by bringing clinicians and geneticists on one platform, thereby influencing the individualized clinical decision making and leading to precision medicine.

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.