Development of the proepicardial organ in the zebrafish

Cardiac Fibroblasts: Helping or Hurting

Cardiac fibroblasts (CFs) are the essential cell type for heart morphogenesis and homeostasis. In addition to maintaining the structural integrity of the heart tissue, muscle fibroblasts are involved in complex signaling cascades that regulate cardiomyocyte proliferation, migration, and maturation. While CFs serve as the primary source of extracellular matrix proteins (ECM), tissue repair, and paracrine signaling, they are also responsible for adverse pathological changes associated with cardiovascular disease. Following activation, fibroblasts produce excessive ECM components that ultimately lead to fibrosis and cardiac dysfunction. Decades of research have led to a much deeper understanding of the role of CFs in cardiogenesis. Recent studies using the single-cell genomic approach have focused on advancing the role of CFs in cellular interactions, and the mechanistic implications involved during cardiovascular development and disease. Arguably, the unique role of fibroblasts in development, tissue repair, and disease progression categorizes them into the friend or foe category. This brief review summarizes the current understanding of cardiac fibroblast biology and discusses the key findings in the context of development and pathophysiological conditions.

The pericardium forms as a distinct structure during heart formation

The heart integrates diverse cell lineages into a functional unit, including the pericardium, a mesothelial sac that supports heart movement, homeostasis, and immune responses. However, despite its critical roles, the developmental origins of the pericardium remain uncertain due to disparate models. Here, using live imaging, lineage tracking, and single-cell transcriptomics in zebrafish, we find the pericardium forms within the lateral plate mesoderm from dedicated anterior mesothelial progenitors and distinct from the classic heart field. Imaging of transgenic reporters in zebrafish documents lateral plate mesoderm cells that emerge lateral of the classic heart field and among a continuous mesothelial progenitor field. Single-cell transcriptomics and trajectories of hand2-expressing lateral plate mesoderm reveal distinct populations of mesothelial and cardiac precursors, including pericardial precursors that are distinct from the cardiomyocyte lineage. The mesothelial gene expression signature is conserved in mammals and carries over to post-natal development. Light sheet-based live-imaging and machine learning-supported cell tracking documents that during heart tube formation, pericardial precursors that reside at the anterior edge of the heart field migrate anteriorly and medially before fusing, enclosing the embryonic heart to form a single pericardial cavity. Pericardium formation proceeds even upon genetic disruption of heart tube formation, uncoupling the two structures. Canonical Wnt/β-catenin signaling modulates pericardial cell number, resulting in a stretched pericardial epithelium with reduced cell number upon canonical Wnt inhibition. We connect the pathological expression of secreted Wnt antagonists of the SFRP family found in pediatric dilated cardiomyopathy to increased pericardial stiffness: sFRP1 in the presence of increased catecholamines causes cardiomyocyte stiffness in neonatal rats as measured by atomic force microscopy. Altogether, our data integrate pericardium formation as an independent process into heart morphogenesis and connect disrupted pericardial tissue properties such as pericardial stiffness to pediatric cardiomyopathies.

Epicardial EMT and cardiac repair: an update

Epicardial epithelial-to-mesenchymal transition (EMT) plays a pivotal role in both heart development and injury response and involves dynamic cellular changes that are essential for cardiogenesis and myocardial repair. Specifically, epicardial EMT is a crucial process in which epicardial cells lose polarity, migrate into the myocardium, and differentiate into various cardiac cell types during development and repair. Importantly, following EMT, the epicardium becomes a source of paracrine factors that support cardiac growth at the last stages of cardiogenesis and contribute to cardiac remodeling after injury. As such, EMT seems to represent a fundamental step in cardiac repair. Nevertheless, endogenous EMT alone is insufficient to stimulate adequate repair. Redirecting and amplifying epicardial EMT pathways offers promising avenues for the development of innovative therapeutic strategies and treatment approaches for heart disease. In this review, we present a synthesis of recent literature highlighting the significance of epicardial EMT reactivation in adult heart disease patients.

Llgl1 mediates timely epicardial emergence and establishment of an apical laminin sheath around the trabeculating cardiac ventricle

During heart development, the embryonic ventricle becomes enveloped by the epicardium, which adheres to the outer apical surface of the heart. This is concomitant with onset of ventricular trabeculation, where a subset of cardiomyocytes lose apicobasal polarity and delaminate basally from the ventricular wall. Llgl1 regulates the formation of apical cell junctions and apicobasal polarity, and we investigated its role in ventricular wall maturation. We found that llgl1 mutant zebrafish embryos exhibit aberrant apical extrusion of ventricular cardiomyocytes. While investigating apical cardiomyocyte extrusion, we identified a basal-to-apical shift in laminin deposition from the internal to the external ventricular wall. We find that epicardial cells express several laminin subunits as they adhere to the ventricle, and that the epicardium is required for laminin deposition on the ventricular surface. In llgl1 mutants, timely establishment of the epicardial layer is disrupted due to delayed emergence of epicardial cells, resulting in delayed apical deposition of laminin on the ventricular surface. Together, our analyses reveal an unexpected role for Llgl1 in correct timing of epicardial development, supporting integrity of the ventricular myocardial wall.

scRNA-seq reveals the diversity of the developing cardiac cell lineage and molecular players in heart rhythm regulation

We utilized scRNA-seq to delineate the diversity of cell types in the zebrafish heart. Transcriptome profiling of over 50,000 cells at 48 and 72 hpf defined at least 18 discrete cell lineages of the developing heart. Utilizing well-established gene signatures, we identified a population of cells likely to be the primary pacemaker and characterized the transcriptome profile defining this critical cell type. Two previously uncharacterized genes, atp1b3b and colec10, were found to be enriched in the sinoatrial cardiomyocytes. CRISPR/Cas9-mediated knockout of these two genes significantly reduced heart rate, implicating their role in cardiac development and conduction. Additionally, we describe other cardiac cell lineages, including the endothelial and neural cells, providing their expression profiles as a resource. Our results established a detailed atlas of the developing heart, providing valuable insights into cellular and molecular mechanisms, and pinpointed potential new players in heart rhythm regulation.

Control of cardiac contractions using Cre-lox and degron strategies in zebrafish

Cardiac contractions and hemodynamic forces are essential for organ development and homeostasis. Control over cardiac contractions can be achieved pharmacologically or optogenetically. However, these approaches lack specificity or require direct access to the heart. Here, we compare two genetic approaches to control cardiac contractions by modulating the levels of the essential sarcomeric protein Tnnt2a in zebrafish. We first recombine a newly generated tnnt2a floxed allele using multiple lines expressing Cre under the control of cardiomyocyte-specific promoters, and show that it does not recapitulate the tnnt2a/silent heart mutant phenotype in embryos. We show that this lack of early cardiac contraction defects is due, at least in part, to the long half-life of tnnt2a mRNA, which masks the gene deletion effects until the early larval stages. We then generate an endogenous Tnnt2a-eGFP fusion line that we use together with the zGRAD system to efficiently degrade Tnnt2a in all cardiomyocytes. Using single-cell transcriptomics, we find that Tnnt2a depletion leads to cardiac phenotypes similar to those observed in tnnt2a mutants, with a loss of blood and pericardial flow-dependent cell types. Furthermore, we achieve conditional degradation of Tnnt2a-eGFP by splitting the zGRAD protein into two fragments that, when combined with the cpFRB2-FKBP system, can be reassembled upon rapamycin treatment. Thus, this Tnnt2a degradation line enables non-invasive control of cardiac contractions with high spatial and temporal specificity and will help further understand how they shape organ development and homeostasis.

Novel Insights into the Molecular Mechanisms Governing Embryonic Epicardium Formation

The embryonic epicardium originates from the proepicardium, an extracardiac primordium constituted by a cluster of mesothelial cells. In early embryos, the embryonic epicardium is characterized by a squamous cell epithelium resting on the myocardium surface. Subsequently, it invades the subepicardial space and thereafter the embryonic myocardium by means of an epithelial-mesenchymal transition. Within the myocardium, epicardial-derived cells present multilineage potential, later differentiating into smooth muscle cells and contributing both to coronary vasculature and cardiac fibroblasts in the mature heart. Over the last decades, we have progressively increased our understanding of those cellular and molecular mechanisms driving proepicardial/embryonic epicardium formation. This study provides a state-of-the-art review of the transcriptional and emerging post-transcriptional mechanisms involved in the formation and differentiation of the embryonic epicardium.

The developing epicardium regulates cardiac chamber morphogenesis by promoting cardiomyocyte growth

The epicardium, the outermost layer of the heart, is an important regulator of cardiac regeneration. However, a detailed understanding of the crosstalk between the epicardium and myocardium during development requires further investigation. Here, we generated three models of epicardial impairment in zebrafish by mutating the transcription factor genes tcf21 and wt1a, and ablating tcf21+ epicardial cells. Notably, all three epicardial impairment models exhibited smaller ventricles. We identified the initial cause of this phenotype as defective cardiomyocyte growth, resulting in reduced cell surface and volume. This failure of cardiomyocyte growth was followed by decreased proliferation and increased abluminal extrusion. By temporally manipulating its ablation, we show that the epicardium is required to support cardiomyocyte growth mainly during early cardiac morphogenesis. By transcriptomic profiling of sorted epicardial cells, we identified reduced expression of FGF and VEGF ligand genes in tcf21-/- hearts, and pharmacological inhibition of these signaling pathways in wild type partially recapitulated the ventricular growth defects. Taken together, these data reveal distinct roles of the epicardium during cardiac morphogenesis and signaling pathways underlying epicardial-myocardial crosstalk.

Patterns and Crucial Regulation of Alternative Splicing During Early Development in Zebrafish

Many vertebrate genes generate multiple transcript variants that may encode functionally distinct protein isoforms, but the transcriptomes of various developmental stages are poorly defined. Identifying the transcriptome and its regulation during the normal developmental process is the key to deciphering the developmental stage-specific functions of genes. Here we presented a systematic assessment of the temporal alternative splicing (AS) events during the critical development stages to capture the dynamic gene expression changes and AS in zebrafish. An unexpected transcriptome complexity generated by AS was observed during zebrafish development. The patterns of AS events varied substantially among developmental stages despite the similarities in the total proportion of AS genes. We further found that AS afforded substantial functional diversification of genes through the generation of stage-specific AS events from broadly protein-coding genes as an essential developmental regulatory mechanism. Skipped exon (SE) showed the strongest signals among developmental AS (devAS), suggesting that devAS events generated by SE may be necessary for the normal development of zebrafish. Most developmental genes regulated by AS mechanisms were not modulated in terms of their overall expression levels, indicating that AS shaped the transcriptome independently from transcriptional regulation during development. 128-cell stage was a critical stage for gene transcription during embryonic development. Splicing factors as an essential developmental regulator underwent AS in the potential autoregulatory feedback loop and expressed multiple isoforms. Thus, zebrafish development was shaped by an interplay of programs controlling gene expression levels and AS. Overall, we provided a global view of developmental patterns of AS during zebrafish development and revealed that AS transitions were the crucial regulatory component of zebrafish embryonic development.

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.

Wt1 transcription factor impairs cardiomyocyte specification and drives a phenotypic switch from myocardium to epicardium

During development, the heart grows by addition of progenitor cells to the poles of the primordial heart tube. In the zebrafish, Wilms tumor 1 transcription factor a (wt1a) and b (wt1b) genes are expressed in the pericardium, at the venous pole of the heart. From this pericardial layer, the proepicardium emerges. Proepicardial cells are subsequently transferred to the myocardial surface and form the epicardium, covering the myocardium. We found that while wt1a and wt1b expression is maintained in proepicardial cells, it is downregulated in pericardial cells that contribute cardiomyocytes to the developing heart. Sustained wt1b expression in cardiomyocytes reduced chromatin accessibility of specific genomic loci. Strikingly, a subset of wt1a- and wt1b-expressing cardiomyocytes changed their cell-adhesion properties, delaminated from the myocardium and upregulated epicardial gene expression. Thus, wt1a and wt1b act as a break for cardiomyocyte differentiation, and ectopic wt1a and wt1b expression in cardiomyocytes can lead to their transdifferentiation into epicardial-like cells.

Strong as a Hippo's Heart: Biomechanical Hippo Signaling During Zebrafish Cardiac Development

The heart is comprised of multiple tissues that contribute to its physiological functions. During development, the growth of myocardium and endocardium is coupled and morphogenetic processes within these separate tissue layers are integrated. Here, we discuss the roles of mechanosensitive Hippo signaling in growth and morphogenesis of the zebrafish heart. Hippo signaling is involved in defining numbers of cardiac progenitor cells derived from the secondary heart field, in restricting the growth of the epicardium, and in guiding trabeculation and outflow tract formation. Recent work also shows that myocardial chamber dimensions serve as a blueprint for Hippo signaling-dependent growth of the endocardium. Evidently, Hippo pathway components act at the crossroads of various signaling pathways involved in embryonic zebrafish heart development. Elucidating how biomechanical Hippo signaling guides heart morphogenesis has direct implications for our understanding of cardiac physiology and pathophysiology.

Every Beat You Take-The Wilms' Tumor Suppressor WT1 and the Heart

Nearly three decades ago, the Wilms’ tumor suppressor Wt1 was identified as a crucial regulator of heart development. Wt1 is a zinc finger transcription factor with multiple biological functions, implicated in the development of several organ systems, among them cardiovascular structures. This review summarizes the results from many research groups which allowed to establish a relevant function for Wt1 in cardiac development and disease. During development, Wt1 is involved in fundamental processes as the formation of the epicardium, epicardial epithelial-mesenchymal transition, coronary vessel development, valve formation, organization of the cardiac autonomous nervous system, and formation of the cardiac ventricles. Wt1 is further implicated in cardiac disease and repair in adult life. We summarize here the current knowledge about expression and function of Wt1 in heart development and disease and point out controversies to further stimulate additional research in the areas of cardiac development and pathophysiology. As re-activation of developmental programs is considered as paradigm for regeneration in response to injury, understanding of these processes and the molecules involved therein is essential for the development of therapeutic strategies, which we discuss on the example of WT1.

Towards spatio-temporally resolved developmental cardiac gene regulatory networks in zebrafish

Heart formation in the zebrafish involves a rapid, complex series of morphogenetic events in three-dimensional space that spans cardiac lineage specification through to chamber formation and maturation. This process is tightly orchestrated by a cardiac gene regulatory network (GRN), which ensures the precise spatio-temporal deployment of genes critical for heart formation. Alterations of the timing or spatial localisation of gene expression can have a significant impact in cardiac ontogeny and may lead to heart malformations. Hence, a better understanding of the cellular and molecular basis of congenital heart disease relies on understanding the behaviour of cardiac GRNs with precise spatiotemporal resolution. Here, we review the recent technical advances that have expanded our capacity to interrogate the cardiac GRN in zebrafish. In particular, we focus on studies utilising high-throughput technologies to systematically dissect gene expression patterns, both temporally and spatially during heart development.

From Stripes to a Beating Heart: Early Cardiac Development in Zebrafish

The heart is the first functional organ to form during vertebrate development. Congenital heart defects are the most common type of human birth defect, many originating as anomalies in early heart development. The zebrafish model provides an accessible vertebrate system to study early heart morphogenesis and to gain new insights into the mechanisms of congenital disease. Although composed of only two chambers compared with the four-chambered mammalian heart, the zebrafish heart integrates the core processes and cellular lineages central to cardiac development across vertebrates. The rapid, translucent development of zebrafish is amenable to in vivo imaging and genetic lineage tracing techniques, providing versatile tools to study heart field migration and myocardial progenitor addition and differentiation. Combining transgenic reporters with rapid genome engineering via CRISPR-Cas9 allows for functional testing of candidate genes associated with congenital heart defects and the discovery of molecular causes leading to observed phenotypes. Here, we summarize key insights gained through zebrafish studies into the early patterning of uncommitted lateral plate mesoderm into cardiac progenitors and their regulation. We review the central genetic mechanisms, available tools, and approaches for modeling congenital heart anomalies in the zebrafish as a representative vertebrate model.

Recapturing embryonic potential in the adult epicardium: Prospects for cardiac repair

Research into potential targets for cardiac repair encompasses recognition of tissue‐resident cells with intrinsic regenerative properties. The adult vertebrate heart is covered by mesothelium, named the epicardium, which becomes active in response to injury and contributes to repair, albeit suboptimally. Motivation to manipulate the epicardium for treatment of myocardial infarction is deeply rooted in its central role in cardiac formation and vasculogenesis during development. Moreover, the epicardium is vital to cardiac muscle regeneration in lower vertebrate and neonatal mammalian‐injured hearts. In this review, we discuss our current understanding of the biology of the mammalian epicardium in development and injury. Considering present challenges in the field, we further contemplate prospects for reinstating full embryonic potential in the adult epicardium to facilitate cardiac regeneration.

Notch and Bmp signaling pathways act coordinately during the formation of the proepicardium

BACKGROUND

The epicardium is the outer mesothelial layer of the heart. It encloses the myocardium and plays key roles in heart development and regeneration. It derives from the proepicardium (PE), cell clusters that appear in the dorsal pericardium (DP) close to the atrioventricular canal and the venous pole of the heart, and are released into the pericardial cavity. PE cells are advected around the beating heart until they attach to the myocardium. Bmp and Notch signaling influence PE formation, but it is unclear how both signaling pathways interact during this process in the zebrafish.

RESULTS

Here, we show that the developing PE is influenced by Notch signaling derived from the endothelium. Overexpression of the intracellular receptor of notch in the endothelium enhances bmp expression, increases the number of pSmad1/5 positive cells in the DP and PE, and enhances PE formation. On the contrary, pharmacological inhibition of Notch1 impairs PE formation. bmp2b overexpression can rescue loss of PE formation in the presence of a Notch1 inhibitor, but Notch gain-of-function could not recover PE formation in the absence of Bmp signaling.

CONCLUSIONS

Endothelial Notch signaling activates bmp expression in the heart tube, which in turn induces PE cluster formation from the DP layer.

Are microRNAs responsible for cardiac hypertrophy in fish and mammals? What we can learn in the activation process in a zebrafish ex vivo model

Recent studies have correlated dysregulated miRNA expression with diseased hearts. With the aim of developing an easily manipulated experimental model, phenylephrine (PE) was added to cultured zebrafish hearts to study the expression of miR1 and miR133a by qRT-PCR. Both miRs were downregulated, with greater downregulation leading to higher hypertrophy. The involvement of this miRs was confirmed by the in-vivo inoculation of complementary sequences (AmiR1 and AmiR133a). HSP70 (involved in transporting proteins and in anti-apoptosis processes) was increased in both treatments. Hyperplasia was observed in the epicardium based on WT1 expression (embryonic epicardial cell marker) in both the PE treatment and AmiR133a treatment. The treatment with AmiR1 showed only cardiomyocyte hypertrophy. This ex-vivo model revealed that miR1 and miR133a play a key role in activating early processes leading to myocardium hypertrophy and epicardium hyperplasia and confirmed the expected similarities with hypertrophic disease that occurs in humans.

Analysis of wt1a reporter line expression levels during proepicardium formation in the zebrafish

The epicardium is the outer mesothelial layer of the heart. It covers the myocardium and plays important roles in both heart development and regeneration. It is derived from the proepicardium (PE), groups of cells that emerges at early developmental stages from the dorsal pericardial layer (DP) close to the atrio-ventricular canal and the venous pole of the heart-tube. In zebrafish, PE cells extrude apically into the pericardial cavity as a consequence of DP tissue constriction, a process that is dependent on Bmp pathway signaling. Expression of the transcription factor Wilms tumor-1, Wt1, which is a leader of important morphogenetic events such as apoptosis regulation or epithelial-mesenchymal cell transition, is also necessary during PE formation. In this study, we used the zebrafish model to compare intensity level of the wt1a reporter line epi:GFP in PE and its original tissue, the DP. We found that GFP is present at higher intensity level in the PE tissue, and differentially wt1 expression at pericardial tissues could be involved in the PE formation process. Our results reveal that bmp2b overexpression leads to enhanced GFP level both in DP and in PE tissues.

TETs Regulate Proepicardial Cell Migration through Extracellular Matrix Organization during Zebrafish Cardiogenesis

Ten-eleven translocation (Tet) enzymes (Tet1/2/3) mediate 5-methylcytosine (5mC) hydroxylation, which can facilitate DNA demethylation and thereby impact gene expression. Studied mostly for how mutant isoforms impact cancer, the normal roles for Tet enzymes during organogenesis are largely unknown. By analyzing compound mutant zebrafish, we discovered a requirement for Tet2/3 activity in the embryonic heart for recruitment of epicardial progenitors, associated with development of the atrial-ventricular canal (AVC). Through a combination of methylation, hydroxymethylation, and transcript profiling, the genes encoding the activin A subunit Inhbaa (in endocardium) and Sox9b (in myocardium) were implicated as demethylation targets of Tet2/3 and critical for organization of AVC-localized extracellular matrix (ECM), facilitating migration of epicardial progenitors onto the developing heart tube. This study elucidates essential DNA demethylation modifications that govern gene expression changes during cardiac development with striking temporal and lineage specificities, highlighting complex interactions in multiple cell populations during development of the vertebrate heart.

Lan et al. show that zebrafish larvae mutant for tet2 and tet3 fail to demethylate genes encoding Inhbaa (in endocardium) and Sox9b (in myocardium), leading to defects in ECM needed to form valves and to recruit epicardial progenitors onto the heart tube.

Functional Heterogeneity within the Developing Zebrafish Epicardium

The epicardium is essential during cardiac development, homeostasis, and repair, and yet fundamental insights into its underlying cell biology, notably epicardium formation, lineage heterogeneity, and functional cross-talk with other cell types in the heart, are currently lacking. In this study, we investigated epicardial heterogeneity and the functional diversity of discrete epicardial subpopulations in the developing zebrafish heart. Single-cell RNA sequencing uncovered three epicardial subpopulations with specific genetic programs and distinctive spatial distribution. Perturbation of unique gene signatures uncovered specific functions associated with each subpopulation and established epicardial roles in cell adhesion, migration, and chemotaxis as a mechanism for recruitment of leukocytes into the heart. Understanding which mechanisms epicardial cells employ to establish a functional epicardium and how they communicate with other cardiovascular cell types during development will bring us closer to repairing cellular relationships that are disrupted during cardiovascular disease.

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scRNA-seq uncovered 3 developmental epicardial subpopulations (Epi1-3) in the zebrafish

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Epi1-specific gene, tgm2b, regulates the cell numbers in the main epicardial sheet

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Epi2-specific gene, sema3fb, restricts the number of tbx18⁺ cells in the cardiac outflow tract

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Epi3-specific gene, cxcl12a, guides ptprc/CD45⁺ myeloid cells to the developing heart

Epicardium in Heart Development

The epicardium, the outermost tissue layer that envelops all vertebrate hearts, plays a crucial role in cardiac development and regeneration and has been implicated in potential strategies for cardiac repair. The heterogenous cell population that composes the epicardium originates primarily from a transient embryonic cell cluster known as the proepicardial organ (PE). Characterized by its high cellular plasticity, the epicardium contributes to both heart development and regeneration in two critical ways: as a source of progenitor cells and as a critical signaling hub. Despite this knowledge, there are many unanswered questions in the field of epicardial biology, the resolution of which will advance the understanding of cardiac development and repair. We review current knowledge in cross-species epicardial involvement, specifically in relation to lineage specification and differentiation during cardiac development.

Embryotoxicity assessment and efficient removal of naphthalene from water by irradiated graphene aerogels

Naphthalene has remained a challenge how to eradicate it from the water because of its carcinogenic risk to humans. In the present study, naphthalene prominently increased the rates of embryonic mortality and malformation, and decreased the hatchability of zebrafish which have a high developmental similarity to humans. Moreover, multiple-organ toxicity were notably found in naphthalene-treated zebrafish. Here, irradiated graphene aerogel (IGA) was successfully prepared from high-energy electron beam to generate more wrinkles, folds, defects and a strong absorption capability for naphthalene, compared with the non-irradiated graphene aerogel. IGA was outstandingly found to remove naphthalene from the embryo culture medium, and subsequently inhibit the embryotoxicity and maintain tissue integrity by restoring cardiac function, attenuating apoptosis signals, recovering eye morphology and structure, reducing expression of heat shock protein 70 in the tissues and promoting behavioral capacity. Meanwhile, no obvious negative impact of IGA was found in the developing zebrafish from embryo to larvae. Consequently, reduction in the toxicity of naphthalene during zebrafish embryogenesis was mediated by IGA as an advanced strategy.

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.

Actin dynamics and the Bmp pathway drive apical extrusion of proepicardial cells

The epicardium, the outer mesothelial layer enclosing the myocardium, plays key roles in heart development and regeneration. During embryogenesis, the epicardium arises from the proepicardium (PE), a cell cluster that appears in the dorsal pericardium (DP) close to the venous pole of the heart. Little is known about how the PE emerges from the pericardial mesothelium. Using a zebrafish model and a combination of genetic tools, pharmacological agents and quantitative in vivo imaging, we reveal that a coordinated collective movement of DP cells drives PE formation. We found that Bmp signaling and the actomyosin cytoskeleton promote constriction of the DP, which enables PE cells to extrude apically. We provide evidence that cell extrusion, which has been described in the elimination of unfit cells from epithelia and the emergence of hematopoietic stem cells, is also a mechanism for PE cells to exit an organized mesothelium and fulfil their developmental fate to form a new tissue layer, the epicardium.

Summary: Proepicardial cells emerge from the pericardial mesothelium through apical extrusion, a process that depends on BMP signaling and actomyosin rearrangements.

Prediction of the mechanical response of cardiac alternans by using an electromechanical model of human ventricular myocytes

PURPOSE

Although the quantitative analysis of electromechanical alternans is important, previous studies have focused on electrical alternans, and there is a lack quantitative analysis of mechanical alternans at the subcellular level according to various basic cycle lengths (BCLs). Therefore, we used the excitation-contraction (E-C) coupling model of human ventricular cells to quantitatively analyze the mechanical alternans of ventricular cells according to various BCLs.

METHODS

To implement E-C coupling, we used calcium transient data, which is the output data of electrical simulation using the electrophysiological model of human ventricular myocytes, as the input data of mechanical simulation using the contractile myofilament dynamics model. Moreover, we applied various loads on ventricular cells for implementation of isotonic and isometric contraction.

RESULTS

As the BCL was reduced from 1000 to 200 ms at 30 ms increments, mechanical alternans, as well as electrical alternans, were observed. At this time, the myocardial diastolic tension increased, and the contractile ATP consumption rate remained greater than zero even in the resting state. Furthermore, the time of peak tension, equivalent cell length, and contractile ATP consumption rate were all reduced. There are two tendencies that endocardial, mid-myocardial, and epicardial cells have the maximum amplitude of tension and the peak systolic tension begins to appear at a high rate under the isometric condition at a particular BCL.

CONCLUSIONS

We observed mechanical alternans of ventricular myocytes as well as electrical alternans, and identified unstable conditions associated with mechanical alternans. We also determined the amount of BCL given to each ventricular cell to generate stable and high tension state in the case of isometric contraction.

Coronary Vasculature in Cardiac Development and Regeneration

Functional coronary circulation is essential for a healthy heart in warm-blooded vertebrates, and coronary diseases can have a fatal consequence. Despite the growing interest, the knowledge about the coronary vessel development and the roles of new coronary vessel formation during heart regeneration is still limited. It is demonstrated that early revascularization is required for efficient heart regeneration. In this comprehensive review, we first describe the coronary vessel formation from an evolutionary perspective. We further discuss the cell origins of coronary endothelial cells and perivascular cells and summarize the critical signaling pathways regulating coronary vessel development. Lastly, we focus on the current knowledge about the molecular mechanisms regulating heart regeneration in zebrafish, a genetically tractable vertebrate model with a regenerative adult heart and well-developed coronary system.

Proepicardium: Current Understanding of its Structure, Induction, and Fate

The proepicardium (PE) is a transitory extracardiac embryonic structure which plays a crucial role in cardiac morphogenesis and delivers various cell lineages to the developing heart. The PE arises from the lateral plate mesoderm (LPM) and is present in all vertebrate species. During development, mesothelial cells of the PE reach the naked myocardium either as free-floating aggregates in the form of vesicles or via a tissue bridge; subsequently, they attach to the myocardium and, finally, form the third layer of a mature heart-the epicardium. After undergoing epithelial-to-mesenchymal transition (EMT) some of the epicardial cells migrate into the myocardial wall and differentiate into fibroblasts, smooth muscle cells, and possibly other cell types. Despite many recent findings, the molecular pathways that control not only proepicardial induction and differentiation but also epicardial formation and epicardial cell fate are poorly understood. Knowledge about these events is essential because molecular mechanisms that occur during embryonic development have been shown to be reactivated in pathological conditions, for example, after myocardial infarction, during hypertensive heart disease or other cardiovascular diseases. Therefore, in this review we intended to summarize the current knowledge about PE formation and structure, as well as proepicardial cell fate in animals commonly used as models for studies on heart development. Anat Rec, 302:893-903, 2019. © 2018 Wiley Periodicals, Inc.

The epicardium as a hub for heart regeneration

After decades of directed research, no effective regenerative therapy is currently available to repair the injured human heart. The epicardium, a layer of mesothelial tissue that envelops the heart in all vertebrates, has emerged as a new player in cardiac repair and regeneration. The epicardium is essential for muscle regeneration in the zebrafish model of innate heart regeneration, and the epicardium also participates in fibrotic responses in mammalian hearts. This structure serves as a source of crucial cells, such as vascular smooth muscle cells, pericytes, and fibroblasts, during heart development and repair. The epicardium also secretes factors that are essential for proliferation and survival of cardiomyocytes. In this Review, we describe recent advances in our understanding of the biology of the epicardium and the effect of these findings on the candidacy of this structure as a therapeutic target for heart repair and regeneration.

The face of epicardial and endocardial derived cells in zebrafish

Zebrafish hearts can regenerate through activation of growth factors and trans-differentiation of fibroblasts, epicardial, myocardial and endocardial cells, all positive for GATA4 during the process. A possible model of regeneration of the whole heart and the regenerating cells in ex-vivo culture is presented here by a stimulation of cocktail of growth factors. In ex-vivo growth-factors-supplemented culture the heart regeneration was quite complete without signs of fibrosis. Epicardial- and endocardial-derived cells have been analyzed by electron microscopy evidencing two main types: 1) larger/prismatic and 2) small/rounded. Type (1) showed on the surface protein-sculptures, while type(2) was smooth with sparse globular proteins. To confirm their nature we have contemporarily analyzed their proliferative capability and markers-positivity. The cells treated by growth factors have at least two-fold more proliferation with GATA4-positivity. The type (1) cell evidenced WT1+(marker of embryonic epicardium); the type (2) showed NFTA2+(marker of embryonic endocardium); whereas cTNT-cardiotroponin was negative. Under growth factors stimulation, GATA4+/WT1+ and GATA4+/NFTA2+ could be suitable candidates to be the cells with capability to move in/out of the tissue, probably by using their integrins, and it opens the possibility to have long term selected culture to future characterization.

ALPK2 Promotes Cardiogenesis in Zebrafish and Human Pluripotent Stem Cells

Cardiac development requires coordinated biphasic regulation of the WNT/β-catenin signaling pathway. By intersecting gene expression and loss-of-function siRNA screens we identified Alpha Protein Kinase 2 (ALPK2) as a candidate negative regulator of WNT/β-catenin signaling in cardiogenesis. In differentiating human embryonic stem cells (hESCs), ALPK2 is highly induced as hESCs transition from mesoderm to cardiac progenitors. Using antisense knockdown and CRISPR/Cas9 mutagenesis in hESCs and zebrafish, we demonstrate that ALPK2 promotes cardiac function and cardiomyocyte differentiation. Quantitative phosphoproteomics, protein expression profiling, and β-catenin reporter assays demonstrate that loss of ALPK2 led to stabilization of β-catenin and increased WNT signaling. Furthermore, cardiac defects attributed to ALPK2 depletion can be rescued in a dose-dependent manner by direct inhibition of WNT signaling through the small molecule XAV939. Together, these results demonstrate that ALPK2 regulates β-catenin-dependent signaling during developmental commitment of cardiomyocytes.

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ALPK2 is expressed and regulated during hESC cardiomyocyte lineage determination

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Cardiac development in zebrafish embryos and hESCs requires ALPK2

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ALPK2 negatively regulates WNT signaling to promote cardiomyocyte differentiation

The ontogeny, activation and function of the epicardium during heart development and regeneration

The epicardium plays a key role during cardiac development, homeostasis and repair, and has thus emerged as a potential target in the treatment of cardiovascular disease. However, therapeutically manipulating the epicardium and epicardium-derived cells (EPDCs) requires insights into their developmental origin and the mechanisms driving their activation, recruitment and contribution to both the embryonic and adult injured heart. In recent years, studies of various model systems have provided us with a deeper understanding of the microenvironment in which EPDCs reside and emerge into, of the crosstalk between the multitude of cardiovascular cell types that influence the epicardium, and of the genetic programmes that orchestrate epicardial cell behaviour. Here, we review these discoveries and discuss how technological advances could further enhance our knowledge of epicardium-based repair mechanisms and ultimately influence potential therapeutic outcomes in cardiovascular regenerative medicine.

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.

Function and Regulation of the Wilms' Tumor Suppressor 1 (WT1) Gene in Fish

The Wilms' tumor suppressor gene Wt1 is highly conserved among vertebrates. In contrast to mammals, most fish species possess two wt1 paralogs that have been named wt1a and wt1b. Concerning wt1 in fish, most work so far has been done using zebrafish, focusing on the embryonic kidney, the pronephros. In this chapter we will describe the structure and development of the pronephros as well as the role that the wt1 genes play in the embryonic zebrafish kidney. We also discuss Wt1 target genes and describe the potential function of the Wt1 proteins in the adult kidney. Finally we will summarize data on the role of Wt1 outside of the kidney.

Microarray-based bioinformatics analysis of the combined effects of SiNPs and PbAc on cardiovascular system in zebrafish

With rapid development of nanotechnology and growing environmental pollution, the combined toxic effects of SiNPs and pollutants of heavy metals like lead have received global attentions. The aim of this study was to explore the cardiovascular effects of the co-exposure of SiNPs and lead acetate (PbAc) in zebrafish using microarray and bioinformatics analysis. Although there was no other obvious cardiovascular malformation except bleeding phenotype, bradycardia, angiogenesis inhibition and declined cardiac output in zebrafish co-exposed of SiNPs and PbAc at NOAEL level, significant changes were observed in mRNA and microRNA (miRNA) expression patterns. STC-GO analysis indicated that the co-exposure might have more toxic effects on cardiovascular system than that exposure alone. Key differentially expressed genes were discerned out based on the Dynamic-gene-network, including stxbp1a, ndfip2, celf4 and gsk3b. Furthermore, several miRNAs obtained from the miRNA-Gene-Network might play crucial roles in cardiovascular disease, such as dre-miR-93, dre-miR-34a, dre-miR-181c, dre-miR-7145, dre-miR-730, dre-miR-129-5p, dre-miR-19d, dre-miR-218b, dre-miR-221. Besides, the analysis of miRNA-pathway-network indicated that the zebrafish were stimulated by the co-exposure of SiNPs and PbAc, which might cause the disturbance of calcium homeostasis and endoplasmic reticulum stress. As a result, cardiac muscle contraction might be deteriorated. In general, our data provide abundant fundamental research clues to the combined toxicity of environmental pollutants and further in-depth verifications are needed.

Dissection of zebrafish shha function using site-specific targeting with a Cre-dependent genetic switch

Despite the extensive use of zebrafish as a model organism in developmental biology and regeneration research, genetic techniques enabling conditional analysis of gene function are limited. In this study, we generated Zwitch, a Cre-dependent invertible gene-trap cassette, enabling the establishment of conditional alleles in zebrafish by generating intronic insertions via in vivo homologous recombination. To demonstrate the utility of Zwitch, we generated a conditional sonic hedgehog a (shha) allele. Homozygous shha mutants developed normally; however, shha mutant embryos globally expressing Cre exhibited strong reductions in endogenous shha and shha target gene mRNA levels and developmental defects associated with null shha mutations. Analyzing a conditional shha mutant generated using an epicardium-specific inducible Cre driver revealed unique roles for epicardium-derived Shha in myocardial proliferation during heart development and regeneration. Zwitch will extend the utility of zebrafish in organ development and regeneration research and might be applicable to other model organisms.

DOI:http://dx.doi.org/10.7554/eLife.24635.001

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.

TCF21 rs12190287 Polymorphisms Are Associated with Ventricular Septal Defects in a Chinese Population

AIMS

TCF21 knockout mice display cardiac defects, including ventricular septal defects (VSDs). Functional rs12190287 polymorphisms located within the 3' untranslated region (3'-UTR) of TCF21 were associated with a risk of coronary heart disease in the European and Eastern populations. However, whether rs12190287 polymorphisms in the TCF21-3'UTR confer predisposition to congenital heart disease (CHD) is unclear.

METHODS

A case-control study was designed consisting of 781 nonsyndromic VSD patients and 867 non-CHD control subjects. The genotype frequency of rs12190287 polymorphisms was determined by real-time polymerase chain reaction.

RESULTS

There were significant differences in the genotype and allele frequencies of rs12190287 between the cases and controls in a Chinese population. Allele G of rs12190287 was significantly associated with an increased risk of VSD in a Chinese population.

CONCLUSIONS

Our results demonstrate that rs12190287 polymorphisms confer predisposition to VSDs in the Chinese population studied here.

Internal epitope tagging informed by relative lack of sequence conservation

Many experimental techniques rely on specific recognition and stringent binding of proteins by antibodies. This can readily be achieved by introducing an epitope tag. We employed an approach that uses a relative lack of evolutionary conservation to inform epitope tag site selection, followed by integration of the tag-coding sequence into the endogenous locus in zebrafish. We demonstrate that an internal epitope tag is accessible for antibody binding, and that tagged proteins retain wild type function.

Wtip is required for proepicardial organ specification and cardiac left/right asymmetry in zebrafish

Wilm's tumor 1 interacting protein (Wtip) was identified as an interacting partner of Wilm's tumor protein (WT1) in a yeast two-hybrid screen. WT1 is expressed in the proepicardial organ (PE) of the heart, and mouse and zebrafish wt1 knockout models appear to lack the PE. Wtip's role in the heart remains unexplored. In the present study, we demonstrate that wtip expression is identical in wt1a-, tcf21-, and tbx18-positive PE cells, and that Wtip protein localizes to the basal body of PE cells. We present the first genetic evidence that Wtip signaling in conjunction with WT1 is essential for PE specification in the zebrafish heart. By overexpressing wtip mRNA, we observed ectopic expression of PE markers in the cardiac and pharyngeal arch regions. Furthermore, wtip knockdown embryos showed perturbed cardiac looping and lacked the atrioventricular (AV) boundary. However, the chamber-specific markers amhc and vmhc were unaffected. Interestingly, knockdown of wtip disrupts early left-right (LR) asymmetry. Our studies uncover new roles for Wtip regulating PE cell specification and early LR asymmetry, and suggest that the PE may exert non-autonomous effects on heart looping and AV morphogenesis. The presence of cilia in the PE, and localization of Wtip in the basal body of ciliated cells, raises the possibility of cilia-mediated PE signaling in the embryonic heart.

Endogenous Mechanisms of Cardiac Regeneration

Zebrafish possess a remarkable capacity for cardiac regeneration throughout their lifetime, providing a model for investigating endogenous cellular and molecular mechanisms regulating myocardial regeneration. By contrast, adult mammals have an extremely limited capacity for cardiac regeneration, contributing to mortality and morbidity from cardiac diseases such as myocardial infarction and heart failure. However, the viewpoint of the mammalian heart as a postmitotic organ was recently revised based on findings that the mammalian heart contains multiple undifferentiated cell types with cardiogenic potential as well as a robust regenerative capacity during a short period early in life. Although it occurs at an extremely low level, continuous cardiomyocyte turnover has been detected in adult mouse and human hearts, which could potentially be enhanced to restore lost myocardium in damaged human hearts. This review summarizes and discusses recent advances in the understanding of endogenous mechanisms of cardiac regeneration.

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.

Interplay between cardiac function and heart development

Mechanotransduction refers to the conversion of mechanical forces into biochemical or electrical signals that initiate structural and functional remodeling in cells and tissues. The heart is a kinetic organ whose form changes considerably during development and disease. This requires cardiomyocytes to be mechanically durable and able to mount coordinated responses to a variety of environmental signals on different time scales, including cardiac pressure loading and electrical and hemodynamic forces. During physiological growth, myocytes, endocardial and epicardial cells have to adaptively remodel to these mechanical forces. Here we review some of the recent advances in the understanding of how mechanical forces influence cardiac development, with a focus on fluid flow forces. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Integration of Developmental and Environmental Cues in the Heart edited by Marcus Schaub and Hughes Abriel.

The Lhx9-integrin pathway is essential for positioning of the proepicardial organ

The development of the vertebrate embryonic heart occurs by hyperplastic growth as well as the incorporation of cells from tissues outside of the initial heart field. Amongst these tissues is the epicardium, a cell structure that develops from the precursor proepicardial organ on the right side of the septum transversum caudal to the developing heart. During embryogenesis, cells of the proepicardial organ migrate, adhere and envelop the maturing heart, forming the epicardium. The cells of the epicardium then delaminate and incorporate into the heart giving rise to cardiac derivatives, including smooth muscle cells and cardiac fibroblasts. Here, we demonstrate that the LIM homeodomain protein Lhx9 is transiently expressed in Xenopus proepicardial cells and is essential for the position of the proepicardial organ on the septum transversum. Utilizing a small-molecule screen, we found that Lhx9 acts upstream of integrin-paxillin signaling and consistently demonstrate that either loss of Lhx9 or disruption of the integrin-paxillin pathway results in mis-positioning of the proepicardial organ and aberrant deposition of extracellular matrix proteins. This leads to a failure of proepicardial cell migration and adhesion to the heart, and eventual death of the embryo. Collectively, these studies establish a requirement for the Lhx9-integrin-paxillin pathway in proepicardial organ positioning and epicardial formation.

Evolutionary morphology of podocytes and primary urine-producing apparatus

Excretory organs were acquired in the early phase of metazoan evolution, and they play a crucial role in the maintenance of homeostasis of body fluids. In general, these organs consist of two functional components, the primary-urine producing apparatus and the modulating tubule. This basic organization of the excretory organs is conserved among most metazoans. Herein, we present an overview of the morphological evolution of the primary urine-producing apparatus in metazoans and describe the acquisition of the renal glomerulus—a specialized primary urine-producing apparatus—in vertebrates. We also describe the advancement of the glomerular structure and function in higher vertebrates.

Chemokine-guided angiogenesis directs coronary vasculature formation in zebrafish

Interruption of the coronary blood supply severely impairs heart function with often fatal consequences for patients. However, the formation and maturation of these coronary vessels is not fully understood. Here we provide a detailed analysis of coronary vessel development in zebrafish. We observe that coronary vessels form in zebrafish by angiogenic sprouting of arterial cells derived from the endocardium at the atrioventricular canal. Endothelial cells express the CXC-motif chemokine receptor Cxcr4a and migrate to vascularize the ventricle under the guidance of the myocardium-expressed ligand Cxcl12b. cxcr4a mutant zebrafish fail to form a vascular network, whereas ectopic expression of Cxcl12b ligand induces coronary vessel formation. Importantly, cxcr4a mutant zebrafish fail to undergo heart regeneration following injury. Our results suggest that chemokine signaling has an essential role in coronary vessel formation by directing migration of endocardium-derived endothelial cells. Poorly developed vasculature in cxcr4a mutants likely underlies decreased regenerative potential in adults.

Temporal cohesion of the structural, functional and molecular characteristics of the developing zebrafish heart

Heart formation is a complex, dynamic and highly coordinated process of molecular, morphogenetic and functional factors with each interacting and contributing to formation of the mature organ. Cardiac abnormalities in early life can be lethal in mammals but not in the zebrafish embryo which has been widely used to study the developing heart. While early cardiac development in the zebrafish has been well characterized, functional changes during development and how these relate to architectural, cellular and molecular aspects of development have not been well described previously. To address this we have carefully characterised cardiac structure, function, cardiomyocyte proliferation and cardiac-specific gene expression between 48 and 120 hpf in the zebrafish. We show that the zebrafish heart increases in volume and changes shape significantly between 48 and 72 hpf accompanied by a 40% increase in cardiomyocyte number. Between 96 and 120 hpf, while external heart expansion slows, there is rapid formation of a mature and extensive trabecular network within the ventricle chamber. While ejection fraction does not change during the course of development other determinants of contractile function increase significantly particularly between 72 and 96 hpf leading to an increase in cardinal vein blood flow. This study has revealed a number of novel aspects of cardiac developmental dynamics with striking temporal orchestration of structure and function within the first few days of development. These changes are associated with changes in expression of developmental and maturational genes. This study provides important insights into the complex temporal relationship between structure and function of the developing zebrafish heart.

Zebrafish pronephros tubulogenesis and epithelial identity maintenance are reliant on the polarity proteins Prkc iota and zeta

The zebrafish pronephros provides an excellent in vivo system to study the mechanisms of vertebrate nephron development. When and how renal progenitors in the zebrafish embryo undergo tubulogenesis to form nephrons is poorly understood, but is known to involve a mesenchymal to epithelial transition (MET) and the acquisition of polarity. Here, we determined the precise timing of these events in pronephros tubulogenesis. As the ternary polarity complex is an essential regulator of epithelial cell polarity across tissues, we performed gene knockdown studies to assess the roles of the related factors atypical protein kinase C iota and zeta (prkcι, prkcζ). We found that prkcι and prkcζ serve partially redundant functions to establish pronephros tubule epithelium polarity. Further, the loss of prkcι or the combined knockdown of prkcι/ζ disrupted proximal tubule morphogenesis and podocyte migration due to cardiac defects that prevented normal fluid flow to the kidney. Surprisingly, tubule cells in prkcι/ζ morphants displayed ectopic expression of the transcription factor pax2a and the podocyte-associated genes wt1a, wt1b, and podxl, suggesting that prkcι/ζ are needed to maintain renal epithelial identity. Knockdown of genes essential for cardiac contractility and vascular flow to the kidney, such as tnnt2a, or elimination of pronephros fluid output through knockdown of the intraflagellar transport gene ift88, was not associated with ectopic pronephros gene expression, thus suggesting a unique role for prkcι/ζ in maintaining tubule epithelial identity separate from the consequence of disruptions to renal fluid flow. Interestingly, knockdown of pax2a, but not wt1a, was sufficient to rescue ectopic tubule gene expression in prkcι/ζ morphants. These data suggest a model in which the redundant activities of prkcι and prkcζ are essential to establish tubule epithelial polarity and also serve to maintain proper epithelial cell type identity in the tubule by inhibiting pax2a expression. These studies provide a valuable foundation for further analysis of MET during nephrogenesis, and have implications for understanding the pathways that affect nephron epithelial cells during kidney disease and regeneration.

Advances in understanding the mechanism of zebrafish heart regeneration

The adult mammalian heart was once believed to be a post-mitotic organ without any capacity for regeneration, but recent findings have challenged this dogma. A modified view assigns the mammalian heart a measurable capacity for regeneration throughout its lifetime, with the implication that endogenous regenerative capacity can be therapeutically stimulated in the injury setting. Although extremely limited in adult mammals, the natural capacity for organ regeneration is a conserved trait in certain vertebrates. Urodele amphibians and teleosts are well-known examples of such animals that can efficiently regenerate various organs including the heart as adults. By understanding how these animals regenerate a damaged heart, one might obtain valuable insights into how regeneration can be augmented in injured human hearts. Among the regenerative vertebrate models, the teleost zebrafish, Danio rerio, is arguably the best characterized with respect to cardiac regenerative responses. Knowledge is still limited, but a decade of research in this model has led to results that may help to understand how cardiac regeneration is naturally stimulated and maintained. This review surveys recent advances in the field and discusses current understanding of the endogenous mechanisms of cardiac regeneration in zebrafish.

Multiple modes of proepicardial cell migration require heartbeat

Background

The outermost layer of the vertebrate heart, the epicardium, forms from a cluster of progenitor cells termed the proepicardium (PE). PE cells migrate onto the myocardium to give rise to the epicardium. Impaired epicardial development has been associated with defects in valve development, cardiomyocyte proliferation and alignment, cardiac conduction system maturation and adult heart regeneration. Zebrafish are an excellent model for studying cardiac development and regeneration; however, little is known about how the zebrafish epicardium forms.

Results

We report that PE migration occurs through multiple mechanisms and that the zebrafish epicardium is composed of a heterogeneous population of cells. Heterogeneity is first observed within the PE and persists through epicardium formation. Using in vivo imaging, histology and confocal microscopy, we show that PE cells migrate through a cellular bridge that forms between the pericardial mesothelium and the heart. We also observed the formation of PE aggregates on the pericardial surface, which were released into the pericardial cavity. It was previously reported that heartbeat-induced pericardiac fluid advections are necessary for PE cluster formation and subsequent epicardium development. We manipulated heartbeat genetically and pharmacologically and found that PE clusters clearly form in the absence of heartbeat. However, when heartbeat was inhibited the PE failed to migrate to the myocardium and the epicardium did not form. We isolated and cultured hearts with only a few epicardial progenitor cells and found a complete epicardial layer formed. However, pharmacologically inhibiting contraction in culture prevented epicardium formation. Furthermore, we isolated control and silent heart (sih) morpholino (MO) injected hearts prior to epicardium formation (60 hpf) and co-cultured these hearts with “donor” hearts that had an epicardium forming (108 hpf). Epicardial cells from donor hearts migrated on to control but not sih MO injected hearts.

Conclusions

Epicardial cells stem from a heterogeneous population of progenitors, suggesting that the progenitors in the PE have distinct identities. PE cells attach to the heart via a cellular bridge and free-floating cell clusters. Pericardiac fluid advections are not necessary for the development of the PE cluster, however heartbeat is required for epicardium formation. Epicardium formation can occur in culture without normal hydrodynamic and hemodynamic forces, but not without contraction.