Heart repair and regeneration: recent insights from zebrafish studies

Ammonium Persulfate-Loaded Carboxylic Gelatin-Methacrylate Nanoparticles Promote Cardiac Repair by Activating Epicardial Epithelial-Mesenchymal Transition via Autophagy and the mTOR Pathway

Restoring damaged myocardial tissue with therapeutic exogenous cells still has some limitations, such as immunological rejection, immature cardiac properties, risk of tumorigenicity, and a low cell survival rate in the ischemic myocardium microenvironment. Activating the endogenous stem cells with functional biomaterials might overcome these limitations. Research has highlighted the multiple differentiation potential of epicardial cells via epithelial-mesenchymal transition (EMT) in both heart development and cardiac regeneration. In our previous research, a carboxylic gelatin-methacrylate (carbox-GelMA) nanoparticle (NP) was fabricated to carry ammonium persulfate (APS), and APS-loaded carbox-GelMA NPs (NPs/APS) could drive the EMT of MCF-7 cells in vitro and promote cancer cell migration and invasion in vivo. The present study explored the roles of functional NPs/APS in the EMT of Wilms' tumor 1-positive (WT1+) epicardial cells and in the repair of myocardial infarction (MI). The WT1+ epicardial cells transformed into endothelial-like cells after being treated with NPs/APS in vitro, and the cardiac functions were improved significantly after injecting NPs/APS into the infarcted hearts in vivo. Furthermore, simultaneous activation of both autophagy and the mTOR pathway was confirmed during the NPs/APS-induced EMT process in WT1+ epicardial cells. Together, this study highlights the function of NPs/APS in the repair of MI.

The multifaceted nature of endogenous cardiac regeneration

Since the first evidence of cardiac regeneration was observed, almost 50 years ago, more studies have highlighted the endogenous regenerative abilities of several models following cardiac injury. In particular, analysis of cardiac regeneration in zebrafish and neonatal mice has uncovered numerous mechanisms involved in the regenerative process. It is now apparent that cardiac regeneration is not simply achieved by inducing cardiomyocytes to proliferate but requires a multifaceted response involving numerous different cell types, signaling pathways and mechanisms which must all work in harmony in order for regeneration to occur. In this review we will endeavor to highlight a variety of processes that have been identifed as being essential for cardiac regeneration.

Zebrafish as a Model to Study Vascular Elastic Fibers and Associated Pathologies

Many extensible tissues such as skin, lungs, and blood vessels require elasticity to function properly. The recoil of elastic energy stored during a stretching phase is provided by elastic fibers, which are mostly composed of elastin and fibrillin-rich microfibrils. In arteries, the lack of elastic fibers leads to a weakening of the vessel wall with an increased risk to develop cardiovascular defects such as stenosis, aneurysms, and dissections. The development of new therapeutic molecules involves preliminary tests in animal models that recapitulate the disease and whose response to drugs should be as close as possible to that of humans. Due to its superior in vivo imaging possibilities and the broad tool kit for forward and reverse genetics, the zebrafish has become an important model organism to study human pathologies. Moreover, it is particularly adapted to large scale studies, making it an attractive model in particular for the first steps of investigations. In this review, we discuss the relevance of the zebrafish model for the study of elastic fiber-related vascular pathologies. We evidence zebrafish as a compelling alternative to conventional mouse models.

Use of Zebrafish Models to Boost Research in Rare Genetic Diseases

Rare genetic diseases are a group of pathologies with often unmet clinical needs. Even if rare by a single genetic disease (from 1/2000 to 1/more than 1,000,000), the total number of patients concerned account for approximatively 400 million peoples worldwide. Finding treatments remains challenging due to the complexity of these diseases, the small number of patients and the challenge in conducting clinical trials. Therefore, innovative preclinical research strategies are required. The zebrafish has emerged as a powerful animal model for investigating rare diseases. Zebrafish combines conserved vertebrate characteristics with high rate of breeding, limited housing requirements and low costs. More than 84% of human genes responsible for diseases present an orthologue, suggesting that the majority of genetic diseases could be modelized in zebrafish. In this review, we emphasize the unique advantages of zebrafish models over other in vivo models, particularly underlining the high throughput phenotypic capacity for therapeutic screening. We briefly introduce how the generation of zebrafish transgenic lines by gene-modulating technologies can be used to model rare genetic diseases. Then, we describe how zebrafish could be phenotyped using state-of-the-art technologies. Two prototypic examples of rare diseases illustrate how zebrafish models could play a critical role in deciphering the underlying mechanisms of rare genetic diseases and their use to identify innovative therapeutic solutions.

The evolutionary and physiological significance of the Hif pathway in teleost fishes

The hypoxia-inducible factor (HIF) pathway is a key regulator of cellular O2 homeostasis and an important orchestrator of the physiological responses to hypoxia (low O2) in vertebrates. Fish can be exposed to significant and frequent changes in environmental O2, and increases in Hif-α (the hypoxia-sensitive subunit of the transcription factor Hif) have been documented in a number of species as a result of a decrease in O2. Here, we discuss the impact of the Hif pathway on the hypoxic response and the contribution to hypoxia tolerance, particularly in fishes of the cyprinid lineage, which includes the zebrafish (Danio rerio). The cyprinids are of specific interest because, unlike in most other fishes, duplicated paralogs of the Hif-α isoforms arising from a teleost-specific genome duplication event have been retained. Positive selection has acted on the duplicated paralogs of the Hif-α isoforms in some cyprinid sub-families, pointing to adaptive evolutionary change in the paralogs. Thus, cyprinids are valuable models for exploring the evolutionary significance and physiological impact of the Hif pathway on the hypoxic response. Knockout in zebrafish of either paralog of Hif-1α greatly reduces hypoxia tolerance, indicating the importance of both paralogs to the hypoxic response. Here, with an emphasis on the cardiorespiratory system, we focus on the role of Hif-1α in the hypoxic ventilatory response and the regulation of cardiac function. We explore the effects of the duration of the hypoxic exposure (acute, sustained or intermittent) on the impact of Hif-1α on cardiorespiratory function and compare relevant data with those from mammalian systems.

The Role of the Epicardium During Heart Development and Repair

The heart is lined by a single layer of mesothelial cells called the epicardium that provides important cellular contributions for embryonic heart formation. The epicardium harbors a population of progenitor cells that undergo epithelial-to-mesenchymal transition displaying characteristic conversion of planar epithelial cells into multipolar and invasive mesenchymal cells before differentiating into nonmyocyte cardiac lineages, such as vascular smooth muscle cells, pericytes, and fibroblasts. The epicardium is also a source of paracrine cues that are essential for fetal cardiac growth, coronary vessel patterning, and regenerative heart repair. Although the epicardium becomes dormant after birth, cardiac injury reactivates developmental gene programs that stimulate epithelial-to-mesenchymal transition; however, it is not clear how the epicardium contributes to disease progression or repair in the adult. In this review, we will summarize the molecular mechanisms that control epicardium-derived progenitor cell migration, and the functional contributions of the epicardium to heart formation and cardiomyopathy. Future perspectives will be presented to highlight emerging therapeutic strategies aimed at harnessing the regenerative potential of the fetal epicardium for cardiac repair.

Traction forces at the cytokinetic ring regulate cell division and polyploidy in the migrating zebrafish epicardium

Epithelial repair and regeneration are driven by collective cell migration and division. Both cellular functions involve tightly controlled mechanical events, but how physical forces regulate cell division in migrating epithelia is largely unknown. Here we show that cells dividing in the migrating zebrafish epicardium exert large cell-extracellular matrix (ECM) forces during cytokinesis. These forces point towards the division axis and are exerted through focal adhesions that connect the cytokinetic ring to the underlying ECM. When subjected to high loading rates, these cytokinetic focal adhesions prevent closure of the contractile ring, leading to multi-nucleation through cytokinetic failure. By combining a clutch model with experiments on substrates of different rigidity, ECM composition and ligand density, we show that failed cytokinesis is triggered by adhesion reinforcement downstream of increased myosin density. The mechanical interaction between the cytokinetic ring and the ECM thus provides a mechanism for the regulation of cell division and polyploidy that may have implications in regeneration and cancer.

Micro RNAs are involved in activation of epicardium during zebrafish heart regeneration

Zebrafish could be an interesting translational model to understand and improve the post-infarction trial and possible regeneration in humans. The adult zebrafish is able to regenerate efficiently after resecting nearly 20% of the ventricular apex. This process requires the concert activation of the epicardium and endocardium, as well as trans-differentiation of pre-existing cardiomyocytes that together replace the lost tissue. The molecular mechanisms involved in this activation process are not completely clarified. In this work, in order to investigate if the downregulation of these miRNAs (miRs) are linked with the activation of epicardium, the expressions of miR-133a, b and miR-1 during regeneration were analysed. qPCR analyses in whole-heart, or from distinct dissected epicardial cells comparing to regenerative clot (containing cardiomyocytes, fibroblasts and endocardial cells) by a laser-micro-dissector, have indicated that already at 24 h there is a downregulation of miRs: (1) miR-133a and miR-1 in the epicardium and (2) miR-133b and miR-1 in the regenerative clot. All the miRs remain downregulated until 7 days post-surgery. With the aim to visualize the activations of heart component in combination with miRs, we developed immunohistochemistry using antibodies directed against common markers in mammals as well as zebrafish: Wilms tumour 1 (WT1), a marker of epicardium; heat-shock protein 70 (HSP70), a chaperon activated during regeneration; and the Cardiac Troponin T (cTnT), a marker of differentiated cardiomyocytes. All these markers are directly or indirectly linked to the investigated miRs. WT1 and HSP70 strongly marked the regeneration site just at 2–3 days postventricular resection. In coherence, cTnT intensively marked the regenerative portion from 7 days onwards. miRs-1 and -133 (a,b) have been strongly involved in the activation of epicardium and regenerative clot during the regeneration process in zebrafish. This study can be a useful translational model to understand the early epicardial activation in which miRs-133a and miR-1 seem to play a central role as observed in the human heart.

A Low-Cost Pulse Generator for Exacerbating Muscle Fiber Detachment Phenotypes in Zebrafish

Muscle fiber detachment from myoseptal boundaries is a common finding in zebrafish models of muscular dystrophies. In some instances, there is a weakening of the interaction between muscle fiber and myosepta, which is yet to manifest as a fiber detachment phenotype. Therefore, to push the fiber detachment of muscle, mutant fish but not their wild-type siblings, beyond their binding threshold, a series of small electrical pulses can be applied to the larvae to create a maximal force contraction and ultimately fiber detachment. To do this, we built a digital pulse generator which delivers four 8 ms 30 V pulses in quick succession, and it has the advantage over older analog approaches to pulse generation because it improves accuracy and is appreciably less expensive. Our pulse generator significantly increases fiber detachment in the laminin-α2 deficient, congenital muscular dystrophy type 1a (MDC1a) model lama2^-/- fish when compared with controls.

MicroRNA-210-mediated proliferation, survival, and angiogenesis promote cardiac repair post myocardial infarction in rodents

An innovative approach for cardiac regeneration following injury is to induce endogenous cardiomyocyte (CM) cell cycle re-entry. In the present study, CMs from adult rat hearts were isolated and transfected with cel-miR-67 (control) and rno-miR-210. A significant increase in CM proliferation and mono-nucleation were observed in miR-210 group, in addition to a reduction in CM size, multi-nucleation, and cell death. When compared to control, β-catenin and Bcl-2 were upregulated while APC (adenomatous polyposis coli), p16, and caspase-3 were downregulated in miR-210 group. In silico analysis predicted cell cycle inhibitor, APC, as a direct target of miR-210 in rodents. Moreover, compared to control, a significant increase in CM survival and proliferation were observed with siRNA-mediated inhibition of APC. Furthermore, miR-210 overexpressing C57BL/6 mice (210-TG) were used for short-term ischemia/reperfusion study, revealing smaller cell size, increased mono-nucleation, decreased multi-nucleation, and increased CM proliferation in 210-TG hearts in contrast to wild-type (NTG). Likewise, myocardial infarction (MI) was created in adult mice, echocardiography was performed, and the hearts were harvested for immunohistochemistry and molecular studies. Compared to NTG, 210-TG hearts showed a significant increase in CM proliferation, reduced apoptosis, upregulated angiogenesis, reduced infarct size, and overall improvement in cardiac function following MI. β-catenin, Bcl-2, and VEGF (vascular endothelial growth factor) were upregulated while APC, p16, and caspase-3 were downregulated in 210-TG hearts. Overall, constitutive overexpression of miR-210 rescues heart function following cardiac injury in adult mice via promoting CM proliferation, cell survival, and angiogenesis.

KEY MESSAGES

MiRNA-210 transfected adult rat CMs show proliferation and reduced cell death in vitro. Cell cycle inhibitor APC is a target of miR-210. MiR-210 overexpressing (210-TG) mouse hearts show CMs cell cycle re-entry and survival post myocardial injury. 210-TG mice show significant neovascularization and angiogenic potential post myocardial infarction. 210-TG hearts show reduced infarct size following ischemic injury.

Gene expression profiles of fin regeneration in loach (Paramisgurnus dabryanu)

Teleost fins can regenerate accurate position-matched structure and function after amputation. However, we still lack systematic transcriptional profiling and methodologies to understand the molecular basis of fin regeneration. After histological analysis, we established a suppression subtraction hybridization library containing 418 distinct sequences expressed differentially during the process of blastema formation and differentiation in caudal fin regeneration. Genome ontology and comparative analysis of differential distribution of our data and the reference zebrafish genome showed notable subcategories, including multi-organism processes, response to stimuli, extracellular matrix, antioxidant activity, and cell junction function. KEGG pathway analysis allowed the effective identification of relevant genes in those pathways involved in tissue morphogenesis and regeneration, including tight junction, cell adhesion molecules, mTOR and Jak-STAT signaling pathway. From relevant function subcategories and signaling pathways, 78 clones were examined for further Southern-blot hybridization. Then, 17 genes were chosen and characterized using semi-quantitative PCR. Then 4 candidate genes were identified, including F11r, Mmp9, Agr2 and one without a match to any database. After real-time quantitative PCR, the results showed obvious expression changes in different periods of caudal fin regeneration. We can assume that the 4 candidates, likely valuable genes associated with fin regeneration, deserve additional attention. Thus, our study demonstrated how to investigate the transcript profiles with an emphasis on bioinformatics intervention and how to identify potential genes related to fin regeneration processes. The results also provide a foundation or knowledge for further research into genes and molecular mechanisms of fin regeneration.

More than Just a Simple Cardiac Envelope; Cellular Contributions of the Epicardium

The adult pumping heart is formed by distinct tissue layers. From inside to outside, the heart is composed by an internal endothelial layer, dubbed the endocardium, a thick myocardial component which supports the pumping capacity of the heart and exteriorly covered by a thin mesothelial layer named the epicardium. Cardiac insults such as coronary artery obstruction lead to ischemia and thus to an irreversible damage of the myocardial layer, provoking in many cases heart failure and death. Thus, searching for new pathways to regenerate the myocardium is an urgent biomedical need. Interestingly, the capacity of heart regeneration is present in other species, ranging from fishes to neonatal mammals. In this context, several lines of evidences demonstrated a key regulatory role for the epicardial layer. In this manuscript, we provide a state-of-the-art review on the developmental process leading to the formation of the epicardium, the distinct pathways controlling epicardial precursor cell specification and determination and current evidences on the regenerative potential of the epicardium to heal the injured heart.

MAPK/ERK signalling is required for zebrafish cardiac regeneration

OBJECTIVES

To better understand the molecular mechanisms of regeneration and explore the potential signalling pathways as therapeutic targets for heart attacks.

RESULTS

After treatment with the MEK inhibitor AZD6244 upon cardiac injury, the core members in MAPK/ERK signalling-mek and erk-demonstrate elevated expression, and these proteins are deposited at the injury site in zebrafish. pERK is also induced in non-cardiomyocytes near the injury site. Furthermore, the induced expression of a dominant-negative form of MEK1 inhibits zebrafish cardiac regeneration, characterized by increased cardiac fibrosis (a hallmark of regenerative failure), reduced or delayed production of regenerative myocardium, and migration of FLI1⁺ endothelial cells, without direct inhibition of cardiomyocyte proliferation.

CONCLUSION

Appropriate activation of MAPK/ERK signalling is essential for zebrafish cardiac regeneration.

Targeting the hedgehog signaling pathway for cardiac repair and regeneration

The hedgehog (Hh) signaling pathway is involved in the angiogenesis and development of the coronary vasculature in the embryonic heart. Recently, the Hh signal pathway has emerged as an important regulator that can increase cardiomyocyte proliferation, inhibit cardiomyocyte death and apoptosis, recruit endothelial progenitor cell (EPCs) into sites of myocardial ischemia, and direct stem cells to differentiate into cardiac muscle lineage. Experimental studies have tried to target the Hh signaling pathway for cardiac repair and regeneration. The purpose of this review is to discuss the role of the Hh signal pathway in cardiac repair and regeneration as well as the current strategies targeting the Hh signaling pathway and its potential in heart diseases.

Cardiomyocyte proliferation in zebrafish and mammals: lessons for human disease

Cardiomyocytes proliferate profusely during early development and for a brief period after birth in mammals. Within a month after birth, this proliferative capability is dramatically reduced in mammals unlike lower vertebrates where it persists into adult life. The zebrafish, for example, retains the ability to regenerate the apex of the heart following resection by a mechanism predominantly driven by cardiomyocyte proliferation. Differences in proliferative capacity of cardiomyocytes in adulthood between mammals and lower vertebrates are closely liked to ontogenetic or phylogenetic factors. Elucidation of these factors has the potential to provide enormous benefits if they lead to the development of therapeutic strategies that facilitate cardiomyocyte proliferation. In this review, we highlight the differences between Mammalian and Zebrafish cardiomyocytes, which could explain at least in part the different proliferative capacities in these two species. We discuss the advantages of the zebrafish as a model of cardiomyocyte proliferation, particularly at the embryonic stage. We also identify a number of key molecular pathways with potential to reveal key steps in switching cardiomyocytes from a quiescent to a proliferative phenotype.

Recent advancements in understanding endogenous heart regeneration-insights from adult zebrafish and neonatal mice

Enhancing the endogenous regenerative capacity of the mammalian heart is a promising strategy that can lead to potential treatment of injured cardiac tissues. Studies on heart regeneration in zebrafish and neonatal mice have shown that cardiomyocyte proliferation is essential for replenishing myocardium. We will review recent advancements that have demonstrated the importance of Neuregulin 1/ErbB2 and innervation in regulating cardiomyocyte proliferation using both adult zebrafish and neonatal mouse heart regeneration models. Emerging findings suggest that different populations of macrophages and inflammation might contribute to regenerative versus fibrotic responses. Finally, we will discuss variation in the severity of the cardiac injury and size of the wound, which may explain the range of outcomes observed in different injury models.

Cell migration during heart regeneration in zebrafish

Zebrafish possess the remarkable ability to regenerate injured hearts as adults, which contrasts the very limited ability in mammals. Although very limited, mammalian hearts do in fact have measurable levels of cardiomyocyte regeneration. Therefore, elucidating mechanisms of zebrafish heart regeneration would provide information of naturally occurring regeneration to potentially apply to mammalian studies, in addition to addressing this biologically interesting phenomenon in itself. Studies over the past 13 years have identified processes and mechanisms of heart regeneration in zebrafish. After heart injury, pre-existing cardiomyocytes dedifferentiate, enter the cell cycle, and repair the injured myocardium. This process requires interaction with epicardial cells, endocardial cells, and vascular endothelial cells. Epicardial cells envelope the heart, while endocardial cells make up the inner lining of the heart. They provide paracrine signals to cardiomyocytes to regenerate the injured myocardium, which is vascularized during heart regeneration. In addition, accumulating results suggest that local migration of these major cardiac cell types have roles in heart regeneration. In this review, we summarize the characteristics of various heart injury methods used in the research community and regeneration of the major cardiac cell types. Then, we discuss local migration of these cardiac cell types and immune cells during heart regeneration. Developmental Dynamics 245:774-787, 2016. © 2016 Wiley Periodicals, Inc.

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.

Contrast-Enhanced X-Ray Micro-Computed Tomography as a Versatile Method for Anatomical Studies of Adult Zebrafish

One attractive quality of zebrafish as a model organism for biological research is that transparency at early developmental stages allows the optical imaging of cellular and molecular events. However, this advantage cannot be applied to adult zebrafish. In this study, we explored the use of contrast-enhanced X-ray micro-computed tomography (microCT) on adult zebrafish in which the organism was stained with iodine, a simple and economical contrasting agent, after fixation. Tomographic reconstruction of the microCT data allowed the three-dimensional (3D) volumetric analyses of individual organs in adult zebrafish. Adipose tissues showed a higher affinity to iodine and were more strongly contrasted in microCT. As traditional histological techniques often involve dehydration steps that remove tissue lipids, iodine-contrasted microCT offers a convenient method for visualizing fat deposition in fish. Utilizing this advantage, we discovered a transient accumulation of lipids around the heart after ventricular amputation, suggesting a correlation between lipid distribution and heart regeneration. Taken together, microCT is a versatile technique that enables the 3D visualization of zebrafish organs, as well as other fish models, in their anatomical context. This simple method is a valuable new addition to the arsenal of techniques available to this model organism.

Regeneration versus scarring in vertebrate appendages and heart

Injuries to complex human organs, such as the limbs and the heart, result in pathological conditions, for which we often lack adequate treatments. While modern regenerative approaches are based on the transplantation of stem cell-derived cells, natural regeneration in lower vertebrates, such as zebrafish and newts, relies predominantly on the intrinsic plasticity of mature tissues. This property involves local activation of the remaining material at the site of injury to promote cell division, cell migration and complete reproduction of the missing structure. It remains an unresolved question why adult mammals are not equally competent to reactivate morphogenetic programmes. Although organ regeneration depends strongly on the proliferative properties of cells in the injured tissue, it is apparent that various organismic factors, such as innervation, vascularization, hormones, metabolism and the immune system, can affect this process. Here, we focus on a correlation between the regenerative capacity and cellular specialization in the context of functional demands, as illustrated by appendages and heart in diverse vertebrates. Elucidation of the differences between homologous regenerative and non-regenerative tissues from various animal models is essential for understanding the applicability of lessons learned from the study of regenerative biology to clinical strategies for the treatment of injured human organs.

Non-coding RNAs in cardiac regeneration

Developing new therapeutic strategies which could enhance cardiomyocyte regenerative capacity is of significant clinical importance. Though promising, methods to promote cardiac regeneration have had limited success due to the weak regenerative capacity of the adult mammalian heart. Non-coding RNAs (ncRNAs), including microRNAs (miRNAs, miRs) and long non-coding RNAs (lncRNAs), are functional RNA molecules without a protein coding function that have been reported to engage in cardiac regeneration and repair. In light of current regenerative strategies, the regulatory effects of ncRNAs can be categorized as follows: cardiac proliferation, cardiac differentiation, cardiac survival and cardiac reprogramming. miR-590, miR-199a, miR-17-92 cluster, miR302-367 cluster and miR-222 have been reported to promote cardiomyocyte proliferation while miR-1 and miR-133 suppress that. miR-499 and miR-1 promote the differentiation of cardiac progenitors into cardiomyocyte while miR-133 and H19 inhibit that. miR-21, miR-24, miR-221, miR-199a and miR-155 improve cardiac survival while miR-34a, miR-1 and miR-320 exhibit opposite effects. miR-1, miR-133, miR-208 and miR-499 are capable of reprogramming fibroblasts to cardiomyocyte-like cells and miR-284, miR-302, miR-93 , miR-106b and lncRNA-ST8SIA3 are able to enhace cardiac reprogramming. Exploring non-coding RNA-based methods to enhance cardiac regeneration would be instrumental for devising new effective therapies against cardiovascular diseases.

Direct cellular reprogramming for cardiac repair and regeneration

Heart failure is a major cause of morbidity and mortality, characterized by depletion of functioning cardiomyocytes, myocardial remodelling, and impaired contractile function. As the heart has a limited capacity for repair, and current treatments do not reverse myocardial attrition, novel regenerative strategies are imperative. Although cell delivery-based approaches remain promising, in situ reprogramming of endogenous cardiac fibroblasts (which are pathophysiologically implicated in cardiac remodelling) into functional cardiomyocytes may represent an advantageous approach. Several groups report successful in vitro and in vivo reprogramming of murine fibroblasts, using critical transcription factors, microRNA mimics, and small molecules, to cells demonstrating cardiomyocyte-like morphology, gene expression, and spontaneous contraction, which improve cardiac function in post-infarct models. Although proof-of-concept studies demonstrate reprogramming in human fibroblasts, significant barriers to therapeutic reprogramming remain. In this review, we evaluate the current status of reprogramming strategies for cardiac repair, and explore future perspectives within the context of clinical translation.

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.

Cardiomyocyte proliferation in cardiac development and regeneration: a guide to methodologies and interpretations

The newt and the zebrafish have the ability to regenerate many of their tissues and organs including the heart. Thus, a major goal in experimental medicine is to elucidate the molecular mechanisms underlying the regenerative capacity of these species. A wide variety of experiments have demonstrated that naturally occurring heart regeneration relies on cardiomyocyte proliferation. Thus, major efforts have been invested to induce proliferation of mammalian cardiomyocytes in order to improve cardiac function after injury or to protect the heart from further functional deterioration. In this review, we describe and analyze methods currently used to evaluate cardiomyocyte proliferation. In addition, we summarize the literature on naturally occurring heart regeneration. Our analysis highlights that newt and zebrafish heart regeneration relies on factors that are also utilized in cardiomyocyte proliferation during mammalian fetal development. Most of these factors have, however, failed to induce adult mammalian cardiomyocyte proliferation. Finally, our analysis of mammalian neonatal heart regeneration indicates experiments that could resolve conflicting results in the literature, such as binucleation assays and clonal analysis. Collectively, cardiac regeneration based on cardiomyocyte proliferation is a promising approach for improving adult human cardiac function after injury, but it is important to elucidate the mechanisms arresting mammalian cardiomyocyte proliferation after birth and to utilize better assays to determine formation of new muscle mass.

High-frequency dual mode pulsed wave Doppler imaging for monitoring the functional regeneration of adult zebrafish hearts

Adult zebrafish is a well-known small animal model for studying heart regeneration. Although the regeneration of scars made by resecting the ventricular apex has been visualized with histological methods, there is no adequate imaging tool for tracking the functional recovery of the damaged heart. For this reason, high-frequency Doppler echocardiography using dual mode pulsed wave Doppler, which provides both tissue Doppler (TD) and Doppler flow in a same cardiac cycle, is developed with a 30 MHz high-frequency array ultrasound imaging system. Phantom studies show that the Doppler flow mode of the dual mode is capable of measuring the flow velocity from 0.1 to 15 cm s(-1) with high accuracy (p-value = 0.974 > 0.05). In the in vivo study of zebrafish, both TD and Doppler flow signals were simultaneously obtained from the zebrafish heart for the first time, and the synchronized valve motions with the blood flow signals were identified. In the longitudinal study on the zebrafish heart regeneration, the parameters for diagnosing the diastolic dysfunction, for example, E/Em < 10, E/A < 0.14 for wild-type zebrafish, were measured, and the type of diastolic dysfunction caused by the amputation was found to be similar to the restrictive filling. The diastolic function was fully recovered within four weeks post-amputation.

Generation of cardiac progenitor cells through epicardial to mesenchymal transition

The epithelial to mesenchymal transition (EMT) is a biological process that drives the formation of cells involved both in tissue repair and in pathological conditions, including tissue fibrosis and tumor metastasis by providing cancer cells with stem cell properties. Recent findings suggest that EMT is reactivated in the heart following ischemic injury. Specifically, epicardial EMT might be involved in the formation of cardiac progenitor cells (CPCs) that can differentiate into endothelial cells, smooth muscle cells, and, possibly, cardiomyocytes. The identification of mechanisms and signaling pathways governing EMT-derived CPC generation and differentiation may contribute to the development of a more efficient regenerative approach for adult heart repair. Here, we summarize key literature in the field.

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.

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.

Differential regenerative capacity of neonatal mouse hearts after cryoinjury

Neonatal mouse hearts fully regenerate after ventricular resection similar to adult zebrafish. We established cryoinjury models to determine if different types and varying degrees of severity in cardiac injuries trigger different responses in neonatal mouse hearts. In contrast to ventricular resection, neonatal mouse hearts fail to regenerate and show severe impairment of cardiac function post transmural cryoinjury. However, neonatal hearts fully recover after non-transmural cryoinjury. Interestingly, cardiomyocyte proliferation does not significantly increase in neonatal mouse hearts after cryoinjuries. Epicardial activation and new coronary vessel formation occur after cryoinjury. The profibrotic marker PAI-1 is highly expressed after transmural but not non-transmural cryoinjuries, which may contribute to the differential scarring. Our results suggest that regenerative medicine strategies for heart injuries should vary depending on the nature of the injury.

Transcriptional response to cardiac injury in the zebrafish: systematic identification of genes with highly concordant activity across in vivo models

Background

Zebrafish is a clinically-relevant model of heart regeneration. Unlike mammals, it has a remarkable heart repair capacity after injury, and promises novel translational applications. Amputation and cryoinjury models are key research tools for understanding injury response and regeneration in vivo. An understanding of the transcriptional responses following injury is needed to identify key players of heart tissue repair, as well as potential targets for boosting this property in humans.

Results

We investigated amputation and cryoinjury in vivo models of heart damage in the zebrafish through unbiased, integrative analyses of independent molecular datasets. To detect genes with potential biological roles, we derived computational prediction models with microarray data from heart amputation experiments. We focused on a top-ranked set of genes highly activated in the early post-injury stage, whose activity was further verified in independent microarray datasets. Next, we performed independent validations of expression responses with qPCR in a cryoinjury model. Across in vivo models, the top candidates showed highly concordant responses at 1 and 3 days post-injury, which highlights the predictive power of our analysis strategies and the possible biological relevance of these genes. Top candidates are significantly involved in cell fate specification and differentiation, and include heart failure markers such as periostin, as well as potential new targets for heart regeneration. For example, ptgis and ca2 were overexpressed, while usp2a, a regulator of the p53 pathway, was down-regulated in our in vivo models. Interestingly, a high activity of ptgis and ca2 has been previously observed in failing hearts from rats and humans.

Conclusions

We identified genes with potential critical roles in the response to cardiac damage in the zebrafish. Their transcriptional activities are reproducible in different in vivo models of cardiac injury.

Electronic supplementary material

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Regenerative responses after mild heart injuries for cardiomyocyte proliferation in zebrafish

BACKGROUND

The zebrafish heart regenerates after various severe injuries. Common processes of heart regeneration are cardiomyocyte proliferation, activation of epicardial tissue, and neovascularization. In order to further characterize heart regeneration processes, we introduced milder injuries and compared responses to those induced by ventricular apex resection, a widely used injury method. We used scratching of the ventricular surface and puncturing of the ventricle with a fine tungsten needle as injury-inducing techniques.

RESULTS

Scratching the ventricular surface induced subtle cardiomyocyte proliferation and responses of the epicardium. Endothelial cell accumulation was limited to the surface of the heart. Ventricular puncture induced cardiomyocyte proliferation, endocardial and epicardial activation, and neo-vascularization, similar to the resection method. However, the degree of the responses was milder, correlating with milder injury. Sham operation induced epicardial aldh1a2 expression but not tbx18 and WT1.

CONCLUSIONS

Puncturing the ventricle induces responses equivalent to resection at milder degrees in a shorter time frame and can be used as a simple injury model. Scratching the ventricle did not induce heart regeneration and can be used for studying wound responses to epicardium.

Noonan and LEOPARD syndrome Shp2 variants induce heart displacement defects in zebrafish

Germline mutations in PTPN11, encoding Shp2, cause Noonan syndrome (NS) and LEOPARD syndrome (LS), two developmental disorders that are characterized by multiple overlapping symptoms. Interestingly, Shp2 catalytic activity is enhanced by NS mutations and reduced by LS mutations. Defective cardiac development is a prominent symptom of both NS and LS, but how the Shp2 variants affect cardiac development is unclear. Here, we have expressed the most common NS and LS Shp2-variants in zebrafish embryos to investigate their role in cardiac development in vivo. Heart function was impaired in embryos expressing NS and LS variants of Shp2. The cardiac anomalies first occurred during elongation of the heart tube and consisted of reduced cardiomyocyte migration, coupled with impaired leftward heart displacement. Expression of specific laterality markers was randomized in embryos expressing NS and LS variants of Shp2. Ciliogenesis and cilia function in Kupffer's vesicle was impaired, likely accounting for the left/right asymmetry defects. Mitogen-activated protein kinase (MAPK) signaling was activated to a similar extent in embryos expressing NS and LS Shp2 variants. Interestingly, inhibition of MAPK signaling prior to gastrulation rescued cilia length and heart laterality defects. These results suggest that NS and LS Shp2 variant-mediated hyperactivation of MAPK signaling leads to impaired cilia function in Kupffer's vesicle, causing left-right asymmetry defects and defective early cardiac development.

Zebrafish--on the move towards ophthalmological research

Millions of people are affected by visual impairment and blindness globally, and the prevalence of vision loss is likely to increase as we are living longer. However, many ocular diseases remain poorly controlled due to lack of proper understanding of the pathogenesis and the corresponding lack of effective therapies. Consequently, there is a major need for animal models that closely mirror the human eye pathology and at the same time allow higher-throughput drug screening approaches. In this context, zebrafish as an animal model organism not only address these needs but can in many respects reflect the human situation better than the current rodent models. Over the past decade, zebrafish have become an established model to study a variety of human diseases and are more recently becoming a valuable tool for the study of human ophthalmological disorders. Many human ocular diseases such as cataract, glaucoma, diabetic retinopathy, and age-related macular degeneration have already been modelled in zebrafish. In addition, zebrafish have become an attractive model for pre-clinical drug toxicity testing and are now increasingly used by scientists worldwide for the discovery of novel treatment approaches. This review presents the advantages and uses of zebrafish for ophthalmological research.

Cardiac regeneration in model organisms

OPINION STATEMENT

Myocardial infarction is the most common cause of cardiac injury in humans and results in acute loss of large numbers of myocardial cells. Unfortunately, the mammalian heart is unable to replenish the cells that are lost following a myocardial infarction and an eventual progression to heart failure can often occur as a result. Regenerative medicine based approaches are actively being developed; however, a complete blueprint on how mammalian hearts can regenerate is still missing. Knowledge gained from studying animal models, such as zebrafish, newt, and neonatal mice, that can naturally regenerate their hearts after injury have provided an understanding of the molecular mechanisms involved in heart repair and regeneration. This research offers novel strategies to overcome the limited regenerative response observed in human patients.

Igf Signaling is Required for Cardiomyocyte Proliferation during Zebrafish Heart Development and Regeneration

Unlike its mammalian counterpart, the adult zebrafish heart is able to fully regenerate after severe injury. One of the most important events during the regeneration process is cardiomyocyte proliferation, which results in the replacement of lost myocardium. Growth factors that induce cardiomyocyte proliferation during zebrafish heart regeneration remain to be identified. Signaling pathways important for heart development might be reutilized during heart regeneration. IGF2 was recently shown to be important for cardiomyocyte proliferation and heart growth during mid-gestation heart development in mice, although its role in heart regeneration is unknown. We found that expression of igf2b was upregulated during zebrafish heart regeneration. Following resection of the ventricle apex, igf2b expression was detected in the wound, endocardium and epicardium at a time that coincides with cardiomyocyte proliferation. Transgenic zebrafish embryos expressing a dominant negative form of Igf1 receptor (dn-Igf1r) had fewer cardiomyocytes and impaired heart development, as did embryos treated with an Igf1r inhibitor. Moreover, inhibition of Igf1r signaling blocked cardiomyocyte proliferation during heart development and regeneration. We found that Igf signaling is required for a subpopulation of cardiomyocytes marked by gata4:EGFP to contribute to the regenerating area. Our findings suggest that Igf signaling is important for heart development and myocardial regeneration in zebrafish.

Model systems for cardiovascular regenerative biology

There is an urgent clinical need to develop new therapeutic approaches to treat heart failure, but the biology of cardiovascular regeneration is complex. Model systems are required to advance our understanding of biological mechanisms of cardiac regeneration as well as to test therapeutic approaches to regenerate tissue and restore cardiac function following injury. An ideal model system should be inexpensive, easily manipulated, easily reproducible, physiologically representative of human disease, and ethically sound. In this review, we discuss computational, cell-based, tissue, and animal models that have been used to elucidate mechanisms of cardiovascular regenerative biology or to test proposed therapeutic methods to restore cardiac function following disease or injury.

FGF10 Signaling Enhances Epicardial Cell Expansion during Neonatal Mouse Heart Repair

Unlike zebrafish and newt hearts, mammalian hearts have limited capacity to regenerate. Upon injury or disease, the adult mammalian hearts form a fibrotic scar. Recently, it was shown that neonatal mouse hearts can regenerate similarly to adult zebrafish hearts. However, this capacity quickly decreases after postnatal day 7 (P7). Understanding the molecular mechanisms underlying neonatal heart regeneration might lead to therapeutic approaches for regenerating adult mammalian hearts. In this study, we utilized an inducible transgenic mouse model to determine the effects of FGF10 growth factor over expression on neonatal mouse heart regeneration/repair. Over expression of FGF10 in myocardium enhanced the expansion of Wt1 positive epicardial cells at 21 days after heart injury through increased proliferation. However, this expansion of epicardial cells did not lead to increased epithelial-to-mesenchymal transition or affect fibroblast formation or fibrosis, as seen by vimentin expression, after heart injury. Furthermore, neither continuous nor transient expression of FGF10 did not affect scar thickness or length after heart injury in neonatal hearts. Our results suggest that FGF10 can regulate epicardial cell expansion of neonatal mouse hearts after injury; however, FGF10 alone is not sufficient to cause beneficial effects on heart repair.

Neurogenesis in zebrafish - from embryo to adult

Neurogenesis in the developing central nervous system consists of the induction and proliferation of neural progenitor cells and their subsequent differentiation into mature neurons. External as well as internal cues orchestrate neurogenesis in a precise temporal and spatial way. In the last 20 years, the zebrafish has proven to be an excellent model organism to study neurogenesis in the embryo. Recently, this vertebrate has also become a model for the investigation of adult neurogenesis and neural regeneration. Here, we summarize the contributions of zebrafish in neural development and adult neurogenesis.