Zebrafish heart regeneration: 15 years of discoveries

Harnessing the regenerative potential of interleukin11 to enhance heart repair

Balancing between regenerative processes and fibrosis is crucial for heart repair. However, strategies to regulate the balance between these two process are a barrier to the development of effective therapies for heart regeneration. While Interleukin 11 (IL11) is known as a fibrotic factor for the heart, its contribution to heart regeneration remains poorly understood. Here, we uncovered that il11a can initiate robust regenerative programs in the zebrafish heart, including cell cycle reentry of cardiomyocytes (CMs) and coronary expansion, even in the absence of injury. However, the prolonged il11a induction in uninjured hearts causes persistent fibroblast emergence, resulting in cardiac fibrosis. While deciphering the regenerative and fibrotic effects, we found that il11-dependent fibrosis, but not il11-dependent regeneration, is mediated through ERK activity, implying that the dual effects of il11a on regeneration and fibrosis can be uncoupled. To harness the regenerative ability of il11a for injured hearts, we devised a combinatorial treatment through il11a induction with ERK inhibition. Using this approach, we observed enhanced CM proliferation with mitigated fibrosis, achieving a balance between stimulating regenerative processes and curbing fibrotic outcomes. Thus, our findings unveil the mechanistic insights into regenerative roles of il11 signaling, offering the potential therapeutic avenues that utilize a paracrine regenerative factor to foster cardiac repair without exacerbating the fibrotic responses.

Infliximab Limits Injury in Myocardial Infarction

BACKGROUND

The purpose of this study was to investigate a therapeutic approach targeting the inflammatory response and consequent remodeling from ischemic myocardial injury.

METHODS AND RESULTS

Coronary thrombus aspirates were collected from patients at the time of ST-segment-elevation myocardial infarction and subjected to array-based proteome analysis. Clinically indistinguishable at myocardial infarction (MI), patients were stratified into vulnerable and resilient on the basis of 1-year left ventricular ejection fraction and death. Network analysis from coronary aspirates revealed prioritization of tumor necrosis factor-α signaling in patients with worse clinical outcomes. Infliximab, a tumor necrosis factor-α inhibitor, was infused intravenously at reperfusion in a porcine MI model to assess whether infliximab-mediated immune modulation impacts post-MI injury. At 3 days after MI (n=7), infliximab infusion increased proregenerative M2 macrophages in the myocardial border zone as quantified by immunofluorescence (24.1%±23.3% in infliximab versus 9.29%±8.7% in sham; P<0.01). Concomitantly, immunoassays of coronary sinus samples quantified lower troponin I levels (41.72±7.34 pg/mL versus 58.11±10.75 pg/mL; P<0.05) and secreted protein analysis revealed upregulation of injury-modifying interleukin-2, -4, -10, -12, and -18 cytokines in the infliximab-treated cohort. At 4 weeks (n=12), infliximab treatment resulted in significant protective influence, improving left ventricular ejection fraction (53.9%±5.4% versus 36.2%±5.3%; P<0.001) and reducing scar size (8.31%±10.9% versus 17.41%±12.5%; P<0.05).

CONCLUSIONS

Profiling of coronary thrombus aspirates in patients with ST-segment-elevation MI revealed highest association for tumor necrosis factor-α in injury risk. Infliximab-mediated immune modulation offers an actionable pathway to alter MI-induced inflammatory response, preserving contractility and limiting adverse structural remodeling.

Syndecan-4 is required for early-stage repair responses during zebrafish heart regeneration

BACKGROUND

The healing process after a myocardial infarction (MI) in humans involves complex events that replace damaged tissue with a fibrotic scar. The affected cardiac tissue may lose its function permanently. In contrast, zebrafish display a remarkable capacity for scar-free heart regeneration. Previous studies have revealed that syndecan-4 (SDC4) regulates inflammatory response and fibroblast activity following cardiac injury in higher vertebrates. However, whether and how Sdc4 regulates heart regeneration in highly regenerative zebrafish remains unknown.

METHODS AND RESULTS

This study showed that sdc4 expression was differentially regulated during zebrafish heart regeneration by transcriptional analysis. Specifically, sdc4 expression increased rapidly and transiently in the early regeneration phase upon ventricular cryoinjury. Moreover, the knockdown of sdc4 led to a significant reduction in extracellular matrix protein deposition, immune cell accumulation, and cell proliferation at the lesion site. The expression of tgfb1a and col1a1a, as well as the protein expression of Fibronectin, were all down-regulated under sdc4 knockdown. In addition, we verified that sdc4 expression was required for cardiac repair in zebrafish via in vivo electrocardiogram analysis. Loss of sdc4 expression caused an apparent pathological Q wave and ST elevation, which are signs of human MI patients.

CONCLUSIONS

Our findings support that Sdc4 is required to mediate pleiotropic repair responses in the early stage of zebrafish heart regeneration.

Distinct features of the regenerating heart uncovered through comparative single-cell profiling

Adult humans respond to heart injury by forming a permanent scar, yet other vertebrates are capable of robust and complete cardiac regeneration. Despite progress towards characterizing the mechanisms of cardiac regeneration in fish and amphibians, the large evolutionary gulf between mammals and regenerating vertebrates complicates deciphering which cellular and molecular features truly enable regeneration. To better define these features, we compared cardiac injury responses in zebrafish and medaka, two fish species that share similar heart anatomy and common teleost ancestry but differ in regenerative capability. We used single-cell transcriptional profiling to create a time-resolved comparative cell atlas of injury responses in all major cardiac cell types across both species. With this approach, we identified several key features that distinguish cardiac injury response in the non-regenerating medaka heart. By comparing immune responses to injury, we found altered cell recruitment and a distinct pro-inflammatory gene program in medaka leukocytes, and an absence of the injury-induced interferon response seen in zebrafish. In addition, we found a lack of pro-regenerative signals, including nrg1 and retinoic acid, from medaka endothelial and epicardial cells. Finally, we identified alterations in the myocardial structure in medaka, where they lack primordial layer cardiomyocytes and fail to employ a cardioprotective gene program shared by regenerating vertebrates. Our findings reveal notable variation in injury response across nearly all major cardiac cell types in zebrafish and medaka, demonstrating how evolutionary divergence influences the hidden cellular features underpinning regenerative potential in these seemingly similar vertebrates.

Border-zone cardiomyocytes and macrophages contribute to remodeling of the extracellular matrix to promote cardiomyocyte invasion during zebrafish cardiac regeneration

Despite numerous advances in our understanding of zebrafish cardiac regeneration, an aspect that remains less studied is how regenerating cardiomyocytes invade, and eventually replace, the collagen-containing fibrotic tissue following injury. Here, we provide an in-depth analysis of the process of cardiomyocyte invasion using live-imaging and histological approaches. We observed close interactions between protruding cardiomyocytes and macrophages at the wound border zone, and macrophage-deficient irf8 mutant zebrafish exhibited defects in extracellular matrix (ECM) remodeling and cardiomyocyte protrusion into the injured area. Using a resident macrophage ablation model, we show that defects in ECM remodeling at the border zone and subsequent cardiomyocyte protrusion can be partly attributed to a population of resident macrophages. Single-cell RNA-sequencing analysis of cells at the wound border revealed a population of cardiomyocytes and macrophages with fibroblast-like gene expression signatures, including the expression of genes encoding ECM structural proteins and ECM-remodeling proteins. The expression of mmp14b , which encodes a membrane-anchored matrix metalloproteinase, was restricted to cells in the border zone, including cardiomyocytes, macrophages, fibroblasts, and endocardial/endothelial cells. Genetic deletion of mmp14b led to a decrease in 1) macrophage recruitment to the border zone, 2) collagen degradation at the border zone, and 3) subsequent cardiomyocyte invasion. Furthermore, cardiomyocyte-specific overexpression of mmp14b was sufficient to enhance cardiomyocyte invasion into the injured tissue and along the apical surface of the wound. Altogether, our data shed important insights into the process of cardiomyocyte invasion of the collagen-containing injured tissue during cardiac regeneration. They further suggest that cardiomyocytes and resident macrophages contribute to ECM remodeling at the border zone to promote cardiomyocyte replenishment of the fibrotic injured tissue.

The genetics of cardiomyocyte polyploidy

The regulation of ploidy in cardiomyocytes is a complex and tightly regulated aspect of cardiac development and function. Cardiomyocyte ploidy can range from diploid (2N) to 8N or even 16N, and these states change during key stages of development and disease progression. Polyploidization has been associated with cellular hypertrophy to support normal growth of the heart, increased contractile capacity, and improved stress tolerance in the heart. Conversely, alterations to ploidy also occur during cardiac pathogenesis of diseases, such as ischemic and non-ischemic heart failure and arrhythmia. Therefore, understanding which genes control and modulate cardiomyocyte ploidy may provide mechanistic insight underlying cardiac growth, regeneration, and disease. This chapter summarizes the current knowledge regarding the genes involved in the regulation of cardiomyocyte ploidy. We discuss genes that have been directly tested for their role in cardiomyocyte polyploidization, as well as methodologies used to identify ploidy alterations. These genes encode cell cycle regulators, transcription factors, metabolic proteins, nuclear scaffolding, and components of the sarcomere, among others. The general physiological and pathological phenotypes in the heart associated with the genetic manipulations described, and how they coincide with the respective cardiomyocyte ploidy alterations, are further discussed in this chapter. In addition to being candidates for genetic-based therapies for various cardiac maladies, these genes and their functions provide insightful evidence regarding the purpose of widespread polyploidization in cardiomyocytes.

Distinct epicardial gene regulatory programs drive development and regeneration of the zebrafish heart

Unlike the adult mammalian heart, which has limited regenerative capacity, the zebrafish heart fully regenerates following injury. Reactivation of cardiac developmental programs is considered key to successfully regenerating the heart, yet the regulation underlying the response to injury remains elusive. Here, we compared the transcriptome and epigenome of the developing and regenerating zebrafish epicardia. We identified epicardial enhancer elements with specific activity during development or during adult heart regeneration. By generating gene regulatory networks associated with epicardial development and regeneration, we inferred genetic programs driving each of these processes, which were largely distinct. Loss of Hif1ab, Nrf1, Tbx2b, and Zbtb7a, central regulators of the regenerating epicardial network, in injured hearts resulted in elevated epicardial cell numbers infiltrating the wound and excess fibrosis after cryoinjury. Our work identifies differences between the regulatory blueprint deployed during epicardial development and regeneration, underlining that heart regeneration goes beyond the reactivation of developmental programs.

Glycosylation of blood cells during the onset and progression of atherosclerosis and myocardial infarction

Protein glycosylation controls cell-cell and cell-extracellular matrix (ECM) communication in immune, vascular, and inflammatory processes, underlining the critical role of this process in the identification of disease biomarkers and the design of novel therapies. Emerging evidence highlights the critical role of blood cell glycosylation in the pathophysiology of atherosclerosis (ATH) and myocardial infarction (MI). Here, we review the role of glycosylation in the interplay between blood cells, particularly erythrocytes, and endothelial cells (ECs), highlighting the involvement of this critical post/cotranslational modification in settings of cardiovascular disease (CVD). Importantly, we focus on emerging preclinical studies and clinical trials based on glycan-targeted drugs to validate their therapeutic potential. These findings may help establish new trends in preventive medicine and delineate novel targeted therapies in CVD.

Methods for dynamic and whole volume imaging of the zebrafish heart

Tissue development and regeneration are dynamic processes involving complex cell migration and cell-cell interactions. We have developed a protocol for complementary time-lapse and three-dimensional (3D) imaging of tissue for developmental and regeneration studies which we apply here to the zebrafish cardiac vasculature. 3D imaging of fixed specimens is used to first define the subject at high resolution then live imaging captures how it changes dynamically. Hearts from adult and juvenile zebrafish are extracted and cleaned in preparation for the different imaging modalities. For whole-mount 3D confocal imaging, single or multiple hearts with native fluorescence or immuno-labeling are prepared for stabilization or clearing, and then imaged. For live imaging, hearts are placed in a prefabricated fluidic device and set on a temperature-controlled microscope for culture and imaging over several days. This protocol allows complete visualization of morphogenic processes in a 3D context and provides the ability to follow cell behaviors to complement in vivo and fixed tissue studies. This culture and imaging protocol can be applied to different cell and tissue types. Here, we have used it to observe zebrafish coronary vasculature and the migration of coronary endothelial cells during heart regeneration.

An injury-responsive mmp14b enhancer is required for heart regeneration

Mammals have limited capacity for heart regeneration, whereas zebrafish have extraordinary regeneration abilities. During zebrafish heart regeneration, endothelial cells promote cardiomyocyte cell cycle reentry and myocardial repair, but the mechanisms responsible for promoting an injury microenvironment conducive to regeneration remain incompletely defined. Here, we identify the matrix metalloproteinase Mmp14b as an essential regulator of heart regeneration. We identify a TEAD-dependent mmp14b endothelial enhancer induced by heart injury in zebrafish and mice, and we show that the enhancer is required for regeneration, supporting a role for Hippo signaling upstream of mmp14b. Last, we show that MMP-14 function in mice is important for the accumulation of Agrin, an essential regulator of neonatal mouse heart regeneration. These findings reveal mechanisms for extracellular matrix remodeling that promote heart regeneration.

Inter-organ communication during tissue regeneration

Tissue regeneration is not simply a local repair event occurring in isolation from the distant, uninjured parts of the body. Rather, evidence indicates that regeneration is a whole-animal process involving coordinated interactions between different organ systems. Here, we review recent studies that reveal how remote uninjured tissues and organ systems respond to and engage in regeneration. We also discuss the need for toolkits and technological advancements to uncover and dissect organ communication during regeneration.

Zebrafish: A smart tool for heart disease research

The increasing prevalence of heart disease poses a significant threat to human survival and safety. However, the current treatments available for heart disease are quite limited. Therefore, it is of great importance to utilize suitable animal models that can accurately simulate the physiological characteristics of heart disease. This would help improve our understanding of this disease and aid in the development of new treatment methods and drugs. Zebrafish hearts not only exhibit similarities to mammalian hearts, but they also share ~70% of homologous genes with humans. Utilizing zebrafish as an alternative to costly and time-consuming mammalian models offers numerous advantages. Zebrafish models can be easily established and maintained, and compound screening and genetic methods allow for the creation of various economical and easily controlled zebrafish and zebrafish embryonic heart disease models in a short period of time. Consequently, zebrafish have become a powerful tool for exploring the pathological mechanisms of heart disease and identifying new effective genes. In this review, we summarize recent studies on different zebrafish models of heart disease. We also describe the techniques and protocols used to develop zebrafish models of myocardial infarction, heart failure, and congenital heart disease, including surgical procedures, forward and reverse genetics, as well as drug and combination screening. This review aims to promote the utilization of zebrafish models in investigating diverse pathological mechanisms of heart disease, enhancing our knowledge and comprehension of heart disease, and offering novel insights and objectives for exploring the prevention and treatment of heart disease.

YAP/TAZ: Molecular pathway and disease therapy

The Yes-associated protein and its transcriptional coactivator with PDZ-binding motif (YAP/TAZ) are two homologous transcriptional coactivators that lie at the center of a key regulatory network of Hippo, Wnt, GPCR, estrogen, mechanical, and metabolism signaling. YAP/TAZ influences the expressions of downstream genes and proteins as well as enzyme activity in metabolic cycles, cell proliferation, inflammatory factor expression, and the transdifferentiation of fibroblasts into myofibroblasts. YAP/TAZ can also be regulated through epigenetic regulation and posttranslational modifications. Consequently, the regulatory function of these mechanisms implicates YAP/TAZ in the pathogenesis of metabolism-related diseases, atherosclerosis, fibrosis, and the delicate equilibrium between cancer progression and organ regeneration. As such, there arises a pressing need for thorough investigation of YAP/TAZ in clinical settings. In this paper, we aim to elucidate the signaling pathways that regulate YAP/TAZ and explore the mechanisms of YAP/TAZ-induce diseases and their potential therapeutic interventions. Furthermore, we summarize the current clinical studies investigating treatments targeting YAP/TAZ. We also address the limitations of existing research on YAP/TAZ and propose future directions for research. In conclusion, this review aims to provide fresh insights into the signaling mediated by YAP/TAZ and identify potential therapeutic targets to present innovative solutions to overcome the challenges associated with YAP/TAZ.

Comparative Analysis of Heart Regeneration: Searching for the Key to Heal the Heart-Part I: Experimental Injury Models to Study Cardiac Regeneration

Cardiovascular diseases are the leading cause of death worldwide, among which, ischemic heart disease is the most prevalent. Myocardial infarction results from occlusion of a coronary artery, which leads to an insufficient blood supply to the myocardium. As is well known, the massive loss of cardiomyocytes cannot be solved due the limited regenerative ability of the adult mammalian heart. In contrast, some lower vertebrate species can regenerate the heart after injury; their study has disclosed some of the involved cell types, molecular mechanisms and signaling pathways during the regenerative process. In this two-part review, we discuss the current state of the principal response in heart regeneration, where several involved processes are essential for full cardiac function in recovery.

Distinct features of the regenerating heart uncovered through comparative single-cell profiling

Adult humans respond to heart injury by forming a permanent scar, yet other vertebrates are capable of robust and complete cardiac regeneration. Despite progress towards characterizing the mechanisms of cardiac regeneration in fish and amphibians, the large evolutionary gulf between mammals and regenerating vertebrates complicates deciphering which cellular and molecular features truly enable regeneration. To better define these features, we compared cardiac injury responses in zebrafish and medaka, two fish species that share similar heart anatomy and common teleost ancestry but differ in regenerative capability. We used single-cell transcriptional profiling to create a time-resolved comparative cell atlas of injury responses in all major cardiac cell types across both species. With this approach, we identified several key features that distinguish cardiac injury response in the non-regenerating medaka heart. By comparing immune responses to injury, we found altered cell recruitment and a distinct pro-inflammatory gene program in medaka leukocytes, and an absence of the injury-induced interferon response seen in zebrafish. In addition, we found a lack of pro-regenerative signals, including nrg1 and retinoic acid, from medaka endothelial and epicardial cells. Finally, we identified alterations in the myocardial structure in medaka, where they lack embryonic-like primordial layer cardiomyocytes, and fail to employ a cardioprotective gene program shared by regenerating vertebrates. Our findings reveal notable variation in injury response across nearly all major cardiac cell types in zebrafish and medaka, demonstrating how evolutionary divergence influences the hidden cellular features underpinning regenerative potential in these seemingly similar vertebrates.

The Emerging Role of Protein Phosphatase in Regeneration

Maintaining normal cellular behavior is essential for the survival of organisms. One of the main mechanisms to control cellular behavior is protein phosphorylation. The process of protein phosphorylation is reversible under the regulation of protein kinases and protein phosphatases. The importance of kinases in numerous cellular processes has been well recognized. In recent years, protein phosphatases have also been demonstrated to function actively and specifically in various cellular processes and thus have gained more and more attention from researchers. In the animal kingdom, regeneration frequently occurs to replace or repair damaged or missing tissues. Emerging evidence has revealed that protein phosphatases are crucial for organ regeneration. In this review, after providing a brief overview of the classification of protein phosphatases and their functions in several representative developmental processes, we highlight the critical roles that protein phosphatases play in organ regeneration by summarizing the most recent research on the function and underlying mechanism of protein phosphatase in the regeneration of the liver, bone, neuron, and heart in vertebrates.

Single-cell analysis reveals an Angpt4-initiated EPDC-EC-CM cellular coordination cascade during heart regeneration

Mammals exhibit limited heart regeneration ability, which can lead to heart failure after myocardial infarction. In contrast, zebrafish exhibit remarkable cardiac regeneration capacity. Several cell types and signaling pathways have been reported to participate in this process. However, a comprehensive analysis of how different cells and signals interact and coordinate to regulate cardiac regeneration is unavailable. We collected major cardiac cell types from zebrafish and performed high-precision single-cell transcriptome analyses during both development and post-injury regeneration. We revealed the cellular heterogeneity as well as the molecular progress of cardiomyocytes during these processes, and identified a subtype of atrial cardiomyocyte exhibiting a stem-like state which may transdifferentiate into ventricular cardiomyocytes during regeneration. Furthermore, we identified a regeneration-induced cell (RIC) population in the epicardium-derived cells (EPDC), and demonstrated Angiopoietin 4 (Angpt4) as a specific regulator of heart regeneration. angpt4 expression is specifically and transiently activated in RIC, which initiates a signaling cascade from EPDC to endocardium through the Tie2-MAPK pathway, and further induces activation of cathepsin K in cardiomyocytes through RA signaling. Loss of angpt4 leads to defects in scar tissue resolution and cardiomyocyte proliferation, while overexpression of angpt4 accelerates regeneration. Furthermore, we found that ANGPT4 could enhance proliferation of neonatal rat cardiomyocytes, and promote cardiac repair in mice after myocardial infarction, indicating that the function of Angpt4 is conserved in mammals. Our study provides a mechanistic understanding of heart regeneration at single-cell precision, identifies Angpt4 as a key regulator of cardiomyocyte proliferation and regeneration, and offers a novel therapeutic target for improved recovery after human heart injuries.

The Lymphatic Vasculature in Cardiac Development and Ischemic Heart Disease

In recent years, the lymphatic system has received increasing attention due to the fast-growing number of findings about its diverse novel functional roles in health and disease. It is well documented that the lymphatic vasculature plays major roles in the maintenance of tissue-fluid balance, the immune response, and in lipid absorption. However, recent studies have identified an additional growing number of novel and sometimes unexpected functional roles of the lymphatic vasculature in normal and pathological conditions in different organs. Among those, cardiac lymphatics have been shown to play important roles in heart development, ischemic cardiac disease, and cardiac disorders. In this review, we will discuss some of those novel functional roles of cardiac lymphatics, as well as the therapeutic potential of targeting lymphatics for the treatment of cardiovascular diseases.

Regeneration of the heart: from molecular mechanisms to clinical therapeutics

Heart injury such as myocardial infarction leads to cardiomyocyte loss, fibrotic tissue deposition, and scar formation. These changes reduce cardiac contractility, resulting in heart failure, which causes a huge public health burden. Military personnel, compared with civilians, is exposed to more stress, a risk factor for heart diseases, making cardiovascular health management and treatment innovation an important topic for military medicine. So far, medical intervention can slow down cardiovascular disease progression, but not yet induce heart regeneration. In the past decades, studies have focused on mechanisms underlying the regenerative capability of the heart and applicable approaches to reverse heart injury. Insights have emerged from studies in animal models and early clinical trials. Clinical interventions show the potential to reduce scar formation and enhance cardiomyocyte proliferation that counteracts the pathogenesis of heart disease. In this review, we discuss the signaling events controlling the regeneration of heart tissue and summarize current therapeutic approaches to promote heart regeneration after injury.

Cross-species single-cell comparison of systemic and cardiac inflammatory responses after cardiac injury

The immune system coordinates the response to cardiac injury and is known to control regenerative and fibrotic scar outcomes in the heart and subsequent chronic low-grade inflammation associated with heart failure. Here we profiled the inflammatory response to heart injury using single cell transcriptomics to compare and contrast two experimental models with disparate outcomes. We used adult mice, which like humans lack the ability to fully recover and zebrafish which spontaneously regenerate after heart injury. The extracardiac reaction to cardiomyocyte necrosis was also interrogated to assess the specific peripheral tissue and immune cell reaction to chronic stress. Cardiac macrophages are known to play a critical role in determining tissue homeostasis by healing versus scarring. We identified distinct transcriptional clusters of monocytes/macrophages in each species and found analogous pairs in zebrafish and mice. However, the reaction to myocardial injury was largely disparate between mice and zebrafish. The dichotomous response to heart damage between the mammalian and zebrafish monocytes/macrophages may underlie the impaired regenerative process in mice, representing a future therapeutic target.

Molecular mechanisms of anthracycline induced cardiotoxicity: Zebrafish come into play

Anthracyclines are among the most potent chemotherapeutics; however, cardiotoxicity significantly restricts their use. Indeed, anthracycline-induced cardiotoxicity (AIC) fares among the worst types of cardiomyopathy, and may only slowly and partially respond to standard heart failure therapies including β-blockers and ACE inhibitors. No therapy specifically designed to treat anthracycline cardiomyopathy at present, and neither is it known if any such strategy could be developed. To address this gap and to elucidate the molecular basis of AIC with a therapeutic goal in mind, zebrafish has been introduced as an in vivo vertebrate model about a decade ago. Here, we first review our current understanding of the basic molecular and biochemical mechanisms of AIC, and then the contribution of zebrafish to the AIC field. We summarize the generation of embryonic zebrafish AIC models (eAIC) and their use for chemical screening and assessment of genetic modifiers, and then the generation of adult zebrafish AIC models (aAIC) and their use for discovering genetic modifiers via forward mutagenesis screening, deciphering spatial-temporal-specific mechanisms of modifier genes, and prioritizing therapeutic compounds via chemical genetic tools. Several therapeutic target genes and related therapies have emerged, including a retinoic acid (RA)-based therapy for the early phase of AIC and an autophagy-based therapy that, for the first time, is able to reverse cardiac dysfunction in the late phase of AIC. We conclude that zebrafish is becoming an important in vivo model that would accelerate both mechanistic studies and therapeutic development of AIC.

Mesentery AjFGF4-AjFGFR2-ERK pathway modulates intestinal regeneration via targeting cell cycle in echinoderms

OBJECTIVES

The purpose of the study aims to understand the regeneration process and its cytology mechanism in economic echinoderms.

MATERIALS AND METHODS

The intestine regeneration process of Apostichopus japonicus was investigated by immunohistochemistry and the cell proliferation was detected by immunofluorescence and flow cytometry. Fibroblast growth factor 4 of A. japonicus (AjFGF4) was screened by RNA-seq analysis and validated to regulate cell proliferation by siAjFGF4 and recombinant-AjFGF4 treatment. The binding and co-localization of AjFGF4 and AjFGFR2 were verified by Co-IP, GST-pull down, and immunofluorescence. Then, the AjFGF4-AjFGFR2-ERK-cell cycle axis was examined by western blot, immunofluorescence, and flow cytometry techniques.

RESULTS

The mesentery was served as the epicenter of intestinal regeneration via activating cell proliferation and other cellular events. Mechanically, AjFGF4-mediated cell proliferation was dependent on the binding to its receptor AjFGFR2, and then triggered the conserved ERK-MAPK pathway but not JNK and p38 pathway. The activated ERK-MAPK subsequently mediated the expression of cell cycle regulatory proteins of CDK2, Cyclin A, and Cyclin B to promote cell proliferation.

CONCLUSIONS

We provide the first functional evidence that AjFGF4-AjFGFR2-ERK-cell cycle axis mediated cell proliferation was the engine for mesentery-derived intestine regeneration in echinoderms.

The Role of the Notch Signaling Pathway in Recovery of Cardiac Function after Myocardial Infarction

Myocardial infarction (MI) is a pathological process, evidencing as massive death of cardiomyocytes associated with hypoxic and oxidative stress. The formation of areas of fibrosis ultimately leads to heart failure. There are some mechanisms that contribute to the functional repair of the heart. In most mammals, including humans, the Notch signaling pathway has cardioprotective effects. It is involved in the formation of the heart in embryogenesis and in the restoration of cardiac function after MI due to: (1) reducing oxidative stress; (2) prevention of apoptosis; (3) regulation of inflammation; (4) containment of fibrosis and hypertrophy of cardiomyocytes; (5) tissue revascularization; and (6) regulation of proliferation and differentiation of cardiomyocytes. In addition, the Notch signaling pathway interacts with other signaling cascades involved in the pathogenesis of MI and subsequent cardiac repair. In this review, we consider the Notch signaling pathway as a potential target for therapeutic approaches aimed at improving cardiac recovery after MI.

Origin and function of activated fibroblast states during zebrafish heart regeneration

The adult zebrafish heart has a high capacity for regeneration following injury. However, the composition of the regenerative niche has remained largely elusive. Here, we dissected the diversity of activated cell states in the regenerating zebrafish heart based on single-cell transcriptomics and spatiotemporal analysis. We observed the emergence of several transient cell states with fibroblast characteristics following injury, and we outlined the proregenerative function of collagen-12-expressing fibroblasts. To understand the cascade of events leading to heart regeneration, we determined the origin of these cell states by high-throughput lineage tracing. We found that activated fibroblasts were derived from two separate sources: the epicardium and the endocardium. Mechanistically, we determined Wnt signalling as a regulator of the endocardial fibroblast response. In summary, our work identifies specialized activated fibroblast cell states that contribute to heart regeneration, thereby opening up possible approaches to modulating the regenerative capacity of the vertebrate heart.

Single-cell RNA sequencing and spatiotemporal analysis of the regenerating zebrafish heart identify transient proregenerative fibroblast-like cells that are derived from the epicardium and the endocardium. Wnt signalling regulates the endocardial fibroblast response.

Hooked on heart regeneration: the zebrafish guide to recovery

While humans lack sufficient capacity to undergo cardiac regeneration following injury, zebrafish can fully recover from a range of cardiac insults. Over the past two decades, our understanding of the complexities of both the independent and co-ordinated injury responses by multiple cardiac tissues during zebrafish heart regeneration has increased exponentially. Although cardiomyocyte regeneration forms the cornerstone of the reparative process in the injured zebrafish heart, recent studies have shown that this is dependent on prior neovascularization and lymphangiogenesis, which in turn require epicardial, endocardial, and inflammatory cell signalling within an extracellular milieu that is optimized for regeneration. Indeed, it is the amalgamation of multiple regenerative systems and gene regulatory patterns that drives the much-heralded success of the adult zebrafish response to cardiac injury. Increasing evidence supports the emerging paradigm that developmental transcriptional programmes are re-activated during adult tissue regeneration, including in the heart, and the zebrafish represents an optimal model organism to explore this concept. In this review, we summarize recent advances from the zebrafish cardiovascular research community with novel insight into the mechanisms associated with endogenous cardiovascular repair and regeneration, which may be of benefit to inform future strategies for patients with cardiovascular disease.

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

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

Glucocorticoids rapidly promote YAP phosphorylation via the cAMP-PKA pathway to repress mouse cardiomyocyte proliferative potential

Adult mammalian cardiomyocytes (CMs) lose their proliferative potential due to cell-cycle withdrawal and polyploidization and fail to mount a proliferative response to regenerate new CMs after cardiac injury. The decline in the proliferative potential of mammalian CMs occurs in the neonatal period when the endocrine system undergoes drastic changes for adaptation to extra-uterine life. There is an increase in circulating glucocorticoid (GC) levels shortly after birth in mammals, and thus, we sought to determine the roles and mechanisms of GCs in regulating CM proliferation. Here, we showed that GCs suppressed CM proliferation in vitro and in vivo, decreased the total number of CMs, and increased the cross-sectional area of CMs. However, the glucocorticoid receptor antagonist had no effect on CM proliferation. Agonists of adenylate cyclase and protein kinase A (PKA) inhibited CM proliferation, while PKA antagonists or knockdown of PKA alleviated the inhibitory effect of GCs on CM proliferation. GCs and the activation of the cyclic adenosine monophosphate (cAMP)/PKA signaling pathway facilitated yes-associated protein (YAP) phosphorylation in mouse CMs and promoted YAP protein translocation from the nucleus to the cytoplasm. Meanwhile, blocking the cAMP/PKA signaling pathway partially blocked the effect of GCs on YAP protein phosphorylation and YAP protein translocation. Thus, our findings suggest that GCs suppress mouse CM proliferation in vitro and in vivo, through a mechanism that involves targeting the cAMP-PKA-YAP signaling pathway.

Enhancer selection dictates gene expression responses in remote organs during tissue regeneration

Acute trauma stimulates local repair mechanisms but can also impact structures distant from the injury, for example through the activity of circulating factors. To study the responses of remote tissues during tissue regeneration, we profiled transcriptomes of zebrafish brains after experimental cardiac damage. We found that the transcription factor gene cebpd was upregulated remotely in brain ependymal cells as well as kidney tubular cells, in addition to its local induction in epicardial cells. cebpd mutations altered both local and distant cardiac injury responses, altering the cycling of epicardial cells as well as exchange between distant fluid compartments. Genome-wide profiling and transgenesis identified a hormone-responsive enhancer near cebpd that exists in a permissive state, enabling rapid gene expression in heart, brain and kidney after cardiac injury. Deletion of this sequence selectively abolished cebpd induction in remote tissues and disrupted fluid regulation after injury, without affecting its local cardiac expression response. Our findings suggest a model to broaden gene function during regeneration in which enhancer regulatory elements define short- and long-range expression responses to injury.

The Cardiac Sarcomere and Cell Cycle

PURPOSE OF REVIEW

The lack of adult human cardiomyocyte proliferative capacity impairs cardiac regeneration such as after myocardial injury. The sarcomere, a specialized actin cytoskeletal structure that is essential for twitch contraction in cardiomyocytes, has been considered a critical factor limiting adult human cardiomyocyte proliferation through incompletely understood mechanisms.

RECENT FINDINGS

This review summarizes known and emerging regulatory mechanisms connecting the human cardiomyocyte sarcomere to cell cycle regulation including structural and signaling mechanisms. Cardiac regeneration could be augmented through targeting the inhibitory effects of the sarcomere on cardiomyocyte proliferation.

Regenerating vascular mural cells in zebrafish fin blood vessels are not derived from pre-existing mural cells and differentially require Pdgfrb signalling for their development

Vascular networks comprise endothelial cells and mural cells, which include pericytes and smooth muscle cells. To elucidate the mechanisms controlling mural cell recruitment during development and tissue regeneration, we studied zebrafish caudal fin arteries. Mural cells colonizing arteries proximal to the body wrapped around them, whereas those in more distal regions extended protrusions along the proximo-distal vascular axis. Both cell populations expressed platelet-derived growth factor receptor β (pdgfrb) and the smooth muscle cell marker myosin heavy chain 11a (myh11a). Most wrapping cells in proximal locations additionally expressed actin alpha2, smooth muscle (acta2). Loss of Pdgfrb signalling specifically decreased mural cell numbers at the vascular front. Using lineage tracing, we demonstrate that precursor cells located in periarterial regions and expressing Pgdfrb can give rise to mural cells. Studying tissue regeneration, we did not find evidence that newly formed mural cells were derived from pre-existing cells. Together, our findings reveal conserved roles for Pdgfrb signalling in development and regeneration, and suggest a limited capacity of mural cells to self-renew or contribute to other cell types during tissue regeneration.

Ruvbl2 Suppresses Cardiomyocyte Proliferation During Zebrafish Heart Development and Regeneration

Cardiomyocyte proliferation is an important source of new myocardium during heart development and regeneration. Consequently, mutations in drivers of cardiomyocyte proliferation cause congenital heart disease, and infarcted human hearts scar because cardiomyocytes exit the cell cycle postnatally. To boost cardiomyocyte proliferation in either setting, critical regulators must be identified. Through an ENU screen in zebrafish, the liebeskummer (lik) mutant was isolated and described as having elevated cardiomyocyte numbers during embryogenesis. The lik mutation results in a three amino acid insertion into Ruvbl2, a highly conserved ATPase. Because both gain- and loss-of-function properties have been described for ruvbl2^lik, it remains unclear whether Ruvbl2 positively or negatively regulates cardiomyocyte proliferation. Here, we demonstrate that Ruvbl2 is a suppressor of cardiomyocyte proliferation during zebrafish heart development and regeneration. First, we confirmed speculation that augmented cardiomyocyte numbers in ruvbl2^lik/lik hearts arise by hyperproliferation. To characterize bona fide ruvbl2 null animals, we created a ruvbl2 locus deletion allele (ruvbl2^Δ). Like ruvbl2^lik/lik mutants, ruvbl2^Δ/Δ and compound heterozygote ruvbl2^lik/Δ animals display ventricular hyperplasia, demonstrating that lik is a loss of function allele and that ruvbl2 represses cardiomyocyte proliferation. This activity is autonomous because constitutive myocardial overexpression of Ruvbl2 is sufficient to suppress cardiomyocyte proliferation in control hearts and rescue the hyperproliferation observed in ruvbl2^Δ/Δ mutant hearts. Lastly, heat-shock inducible overexpression of Ruvbl2 suppresses cardiomyocyte proliferation during heart regeneration and leads to scarring. Together, our data demonstrate that Ruvbl2 functions autonomously as a suppressor of cardiomyocyte proliferation during both zebrafish heart development and adult heart regeneration.

Amino acid primed mTOR activity is essential for heart regeneration

Heart disease is the leading cause of death with no method to repair damaged myocardium due to the limited proliferative capacity of adult cardiomyocytes. Curiously, mouse neonates and zebrafish can regenerate their hearts via cardiomyocyte de-differentiation and proliferation. However, a molecular mechanism of why these cardiomyocytes can re-enter cell cycle is poorly understood. Here, we identify a unique metabolic state that primes adult zebrafish and neonatal mouse ventricular cardiomyocytes to proliferate. Zebrafish and neonatal mouse hearts display elevated glutamine levels, predisposing them to amino-acid-driven activation of TOR, and that TOR activation is required for zebrafish cardiomyocyte regeneration in vivo. Through a multi-omics approach with cellular validation we identify metabolic and mitochondrial changes during the first week of regeneration. These data suggest that regeneration of zebrafish myocardium is driven by metabolic remodeling and reveals a unique metabolic regulator, TOR-primed state, in which zebrafish and mammalian cardiomyocytes are regeneration competent.

Aging Activates the Immune System and Alters the Regenerative Capacity in the Zebrafish Heart

Age-associated organ failure and degenerative diseases have a major impact on human health. Cardiovascular dysfunction has an increasing prevalence with age and is one of the leading causes of death. In contrast to humans, zebrafish have extraordinary regeneration capacities of complex organs including the heart. In addition, zebrafish has recently become a model organism in research on aging. Here, we have compared the ventricular transcriptome as well as the regenerative capacity after cryoinjury of old and young zebrafish hearts. We identified the immune system as activated in old ventricles and found muscle organization to deteriorate upon aging. Our data show an accumulation of immune cells, mostly macrophages, in the old zebrafish ventricle. Those immune cells not only increased in numbers but also showed morphological and behavioral changes with age. Our data further suggest that the regenerative response to cardiac injury is generally impaired and much more variable in old fish. Collagen in the wound area was already significantly enriched in old fish at 7 days post injury. Taken together, these data indicate an ‘inflammaging’-like process in the zebrafish heart and suggest a change in regenerative response in the old.

Modulation of VEGFA Signaling During Heart Regeneration in Zebrafish

Over the last decades, myocardial infarction and heart failure have accounted every year for millions of deaths worldwide. After a coronary occlusion, the lack of blood supply to downstream muscle leads to cell death and scarring. To date, several pro-angiogenic factors have been tested to stimulate reperfusion of the affected myocardium, VEGFA being one of the most extensively studied. Given the unsuccessful outcomes of clinical trials, understanding how cardiac revascularization takes place in models with endogenous regenerative capacity holds the key to devising more efficient therapies. Here, we summarize the main findings on VEGFA's role during cardiac repair and regeneration, with a particular focus on zebrafish as a regenerative model. Moreover, we provide a comprehensive overview of available tools to modulate Vegfa expression and action in zebrafish regeneration studies. Understanding the role of Vegfa during zebrafish heart regeneration may help devise efficient therapies and circumvent current limitations in using VEGFA for therapeutic angiogenesis approaches.

Oxytocin promotes epicardial cell activation and heart regeneration after cardiac injury

Cardiovascular disease (CVD) is one of the leading causes of mortality worldwide, and frequently leads to massive heart injury and the loss of billions of cardiac muscle cells and associated vasculature. Critical work in the last 2 decades demonstrated that these lost cells can be partially regenerated by the epicardium, the outermost mesothelial layer of the heart, in a process that highly recapitulates its role in heart development. Upon cardiac injury, mature epicardial cells activate and undergo an epithelial-mesenchymal transition (EMT) to form epicardium-derived progenitor cells (EpiPCs), multipotent progenitors that can differentiate into several important cardiac lineages, including cardiomyocytes and vascular cells. In mammals, this process alone is insufficient for significant regeneration, but it might be possible to prime it by administering specific reprogramming factors, leading to enhanced EpiPC function. Here, we show that oxytocin (OXT), a hypothalamic neuroendocrine peptide, induces epicardial cell proliferation, EMT, and transcriptional activity in a model of human induced pluripotent stem cell (hiPSC)-derived epicardial cells. In addition, we demonstrate that OXT is produced after cardiac cryoinjury in zebrafish, and that it elicits significant epicardial activation promoting heart regeneration. Oxytocin signaling is also critical for proper epicardium development in zebrafish embryos. The above processes are significantly impaired when OXT signaling is inhibited chemically or genetically through RNA interference. RNA sequencing data suggests that the transforming growth factor beta (TGF-β) pathway is the primary mediator of OXT-induced epicardial activation. Our research reveals for the first time an evolutionary conserved brain-controlled mechanism inducing cellular reprogramming and regeneration of the injured mammalian and zebrafish heart, a finding that could contribute to translational advances for the treatment of cardiac injuries.

Functional coordination of non-myocytes plays a key role in adult zebrafish heart regeneration

Cardiac regeneration occurs primarily through proliferation of existing cardiomyocytes, but also involves complex interactions between distinct cardiac cell types including non-cardiomyocytes (non-CMs). However, the subpopulations, distinguishing molecular features, cellular functions, and intercellular interactions of non-CMs in heart regeneration remain largely unexplored. Using the LIGER algorithm, we assemble an atlas of cell states from 61,977 individual non-CM scRNA-seq profiles isolated at multiple time points during regeneration. This analysis reveals extensive non-CM cell diversity, including multiple macrophage (MC), fibroblast (FB), and endothelial cell (EC) subpopulations with unique spatiotemporal distributions, and suggests an important role for MC in inducing the activated FB and EC subpopulations. Indeed, pharmacological perturbation of MC function compromises the induction of the unique FB and EC subpopulations. Furthermore, we developed computational algorithm Topologizer to map the topological relationships and dynamic transitions between functional states. We uncover dynamic transitions between MC functional states and identify factors involved in mRNA processing and transcriptional regulation associated with the transition. Together, our single-cell transcriptomic analysis of non-CMs during cardiac regeneration provides a blueprint for interrogating the molecular and cellular basis of this process.

Histone deacetylase 1 controls cardiomyocyte proliferation during embryonic heart development and cardiac regeneration in zebrafish

In contrast to mammals, the zebrafish maintains its cardiomyocyte proliferation capacity throughout adulthood. However, neither the molecular mechanisms that orchestrate the proliferation of cardiomyocytes during developmental heart growth nor in the context of regeneration in the adult are sufficiently defined yet. We identified in a forward genetic N-ethyl-N-nitrosourea (ENU) mutagenesis screen the recessive, embryonic-lethal zebrafish mutant baldrian (bal), which shows severely impaired developmental heart growth due to diminished cardiomyocyte proliferation. By positional cloning, we identified a missense mutation in the zebrafish histone deacetylase 1 (hdac1) gene leading to severe protein instability and the loss of Hdac1 function in vivo. Hdac1 inhibition significantly reduces cardiomyocyte proliferation, indicating a role of Hdac1 during developmental heart growth in zebrafish. To evaluate whether developmental and regenerative Hdac1-associated mechanisms of cardiomyocyte proliferation are conserved, we analyzed regenerative cardiomyocyte proliferation after Hdac1 inhibition at the wound border zone in cryoinjured adult zebrafish hearts and we found that Hdac1 is also essential to orchestrate regenerative cardiomyocyte proliferation in the adult vertebrate heart. In summary, our findings suggest an important and conserved role of Histone deacetylase 1 (Hdac1) in developmental and adult regenerative cardiomyocyte proliferation in the vertebrate heart.

Heart disease is one of the most common causes of death in all developed countries. While zebrafish cardiomyocytes are able to proliferate throughout adulthood, mammalian cardiomyocytes lose this ability during early development, and therefore are not capable to replace and renew cardiomyocytes after injury. The underlying mechanisms of cardiomyocyte proliferation are still not completely resolved. Understanding how zebrafish cardiomyocytes preserve their proliferating state, would be a valuable information to foster cardiac regeneration, e.g. after myocardial infarction in patients. Knowledge of the signaling pathways that need to be activated, or deactivated in order to induce cardiomyocyte proliferation after acute or chronic injury will pave the way for the development of genetic and/or pharmacological treatment options. In an ENU-mutagenesis screen, we identified the zebrafish mutant baldrian, which shows reduced embryonic cardiomyocyte proliferation. As genetic cause of the observed phenotype, we identified a missense mutation in the hdac1 gene. By treatment of heart-injured adult fish with the HDAC1 inhibitor Mocetinostat, we were able to show a reduced rate of cardiomyocyte proliferation also in the adult zebrafish heart in vivo, suggesting a role of Hdac1 in embryonic heart growth and adult regenerative cardiomyocyte proliferation in zebrafish.

Zebrafish Fin: Complex Molecular Interactions and Cellular Mechanisms Guiding Regeneration

The zebrafish caudal fin has become a popular model to study cellular and molecular mechanisms of regeneration due to its high regenerative capacity, accessibility for experimental manipulations, and relatively simple anatomy. The formation of a regenerative epidermis and blastema are crucial initial events and tightly regulated. Both the regenerative epidermis and the blastema are highly organized structures containing distinct domains, and several signaling pathways regulate the formation and interaction of these domains. Bone is the major tissue regenerated from the progenitor cells of the blastema. Several cellular mechanisms can provide source cells for blastemal (pre-)osteoblasts, including dedifferentiation of differentiated osteoblasts and de novo formation from other cell types, providing intriguing examples of cellular plasticity. In recent years, omics analyses and single-cell approaches have elucidated genetic and epigenetic regulation, increasing our knowledge of the surprisingly complex coordination of various mechanisms to achieve successful restoration of a seemingly simple structure.

PET/CT Technology in Adult Zebrafish: A Pilot Study Toward Live Longitudinal Imaging

Decades of research have confirmed the beneficial and advantageous use of zebrafish (Danio rerio) as a model of human disease in biomedical studies. Not only are 71% of human genes shared with the zebrafish many of these genes are linked to human diseases. Currently, numerous transgenic and mutant genetic zebrafish lines are now widely available for use in research. Furthermore, zebrafish are relatively inexpensive to maintain compared to rodents. However, a limiting factor to fully utilising adult zebrafish in research is not the fish but the technological imaging tools available. In order to increase the utilisation of adult zebrafish, which are not naturally transparent, requires new imaging approaches. Therefore, this feasibility study: (1) presents an innovative designed PET/CT adult zebrafish imaging platform, capable of maintaining normal aquatic physiology during scanning; (2) assesses the practical aspects of adult zebrafish imaging; and (3) set basic procedural guidelines for zebrafish imaging during a PET/CT acquisition. Methods: With computer aided design (CAD) software an imaging platform was developed for 3D printing. A 3D printed zebrafish model was created from a CT acquisition of a zebrafish using the CAD software. This model and subsequently euthanised zebrafish were imaged post-injection using different concentrations of the radiotracer [18F]FDG with CT contrast. Results: PET/CT imaging was successful, revealing levels as low as 0.01 MBq could be detected in the fish. In the zebrafish imaging post-injection distribution of the radiotracer was observed away from the injection site as well as tissue uptake. Potential preliminary husbandry and welfare guidelines for the fish during and after PET/CT imaging were determined. Conclusion: Using PET/CT for adult zebrafish imaging as a viable non-invasive technological tool is feasible. Adult zebrafish PET/CT imaging has the potential to be a key imaging technique offering the possibilities of enhanced biomedical understanding and new translational data sets.

Advances in Cardiac Development and Regeneration Using Zebrafish as a Model System for High-Throughput Research

Heart disease is the leading cause of death in the United States and worldwide. Understanding the molecular mechanisms of cardiac development and regeneration will improve diagnostic and therapeutic interventions against heart disease. In this direction, zebrafish is an excellent model because several processes of zebrafish heart development are largely conserved in humans, and zebrafish has several advantages as a model organism. Zebrafish transcriptomic profiles undergo alterations during different stages of cardiac development and regeneration which are revealed by RNA-sequencing. ChIP-sequencing has detected genome-wide occupancy of histone post-translational modifications that epigenetically regulate gene expression and identified a locus with enhancer-like characteristics. ATAC-sequencing has identified active enhancers in cardiac progenitor cells during early developmental stages which overlap with occupancy of histone modifications of active transcription as determined by ChIP-sequencing. CRISPR-mediated editing of the zebrafish genome shows how chromatin modifiers and DNA-binding proteins regulate heart development, in association with crucial signaling pathways. Hence, more studies in this direction are essential to improve human health because they answer fundamental questions on cardiac development and regeneration, their differences, and why zebrafish hearts regenerate upon injury, unlike humans. This review focuses on some of the latest studies using state-of-the-art technology enabled by the elegant yet simple zebrafish.

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.

Interleukin-11 signaling promotes cellular reprogramming and limits fibrotic scarring during tissue regeneration

Damage-induced fibrotic scarring limits tissue regeneration in mammals and is a leading cause of morbidity. In contrast, species like zebrafish can regenerate damaged tissues without excessive fibrosis. However, whether specific signaling pathways can both limit fibrosis and promote regeneration is unclear. Here, we show that interleukin-11 (Il-11)/Stat3 signaling has such a dual function. Zebrafish lacking Il-11 receptor function display severely compromised heart, fin, and scale regeneration. Deep phenotyping and transcriptional analysis of adult hearts and fins show that Il-11 signaling drives cellular reprogramming to orchestrate global and tissue-specific regenerative programs and broadly antagonizes hallmarks of adult mammalian scarring. Mechanistically, our data indicate that IL-11 signaling in endothelial cells antagonizes profibrotic transforming growth factor–β signaling and endothelial-to-mesenchymal transition, limiting scarring and promoting cardiomyocyte repopulation, after injury. Overall, our findings position damage-induced Il-11/Stat3 signaling in a key role limiting fibrosis and promoting regeneration, revealing novel targets for regenerative therapies.

Mechanisms Underlying Cardiomyocyte Development: Can We Exploit Them to Regenerate the Heart?

PURPOSE OF REVIEW

It is well established that the adult mammalian cardiomyocytes retain a low capacity for cell cycle activity; however, it is insufficient to effectively respond to myocardial injury and facilitate cardiac regenerative repair. Lessons learned from species in which cardiomyocytes do allow for proliferative regeneration/repair have shed light into the mechanisms underlying cardiac regeneration post-injury. Importantly, many of these mechanisms are conserved across species, including mammals, and efforts to tap into these mechanisms effectively within the adult heart are currently of great interest.

RECENT FINDINGS

Targeting the endogenous gene regulatory networks (GRNs) shown to play roles in the cardiac regeneration of conducive species is seen as a strong approach, as delivery of a single or combination of genes has promise to effectively enhance cell cycle activity and CM proliferation in adult hearts post-myocardial infarction (MI). In situ re-induction of proliferative gene regulatory programs within existing, local, non-damaged cardiomyocytes helps overcome significant technical hurdles, such as successful engraftment of implanted cells or achieving complete cardiomyocyte differentiation from cell-based approaches. Although many obstacles currently exist and need to be overcome to successfully translate these approaches to clinical settings, the current efforts presented here show great promise.

Apex Resection in Zebrafish (Danio rerio) as a Model of Heart Regeneration: A Video-Assisted Guide

Ischemic heart disease is one of the leading causes of deaths worldwide. A major hindrance to resolving this challenge lies in the mammalian hearts inability to regenerate after injury. In contrast, zebrafish retain a regenerative capacity of the heart throughout their lifetimes. Apex resection (AR) is a popular zebrafish model for studying heart regeneration, and entails resecting 10-20% of the heart in the apex region, whereafter the regeneration process is monitored until the heart is fully regenerated within 60 days. Despite this popularity, video tutorials describing this technique in detail are lacking. In this paper we visualize and describe the entire AR procedure including anaesthesia, surgery, and recovery. In addition, we show that the concentration and duration of anaesthesia are important parameters to consider, to balance sufficient levels of sedation and minimizing mortality. Moreover, we provide examples of how zebrafish heart regeneration can be assessed both in 2D (immunohistochemistry of heart sections) and 3D (analyses of whole, tissue cleared hearts using multiphoton imaging). In summary, this paper aims to aid beginners in establishing and conducting the AR model in their laboratory, but also to spur further interest in improving the model and its evaluation.

Tp53 Suppression Promotes Cardiomyocyte Proliferation during Zebrafish Heart Regeneration

Zebrafish regenerate heart muscle through division of pre-existing cardiomyocytes. To discover underlying regulation, we assess transcriptome datasets for dynamic gene networks during heart regeneration and identify suppression of genes associated with the transcription factor Tp53. Cardiac damage leads to fluctuation of Tp53 protein levels, concomitant with induced expression of its central negative regulator, mdm2, in regenerating cardiomyocytes. Zebrafish lacking functional Tp53 display increased indicators of cardiomyocyte proliferation during regeneration, whereas transgenic Mdm2 blockade inhibits injury-induced cardiomyocyte proliferation. Induced myocardial overexpression of the mitogenic factors Nrg1 or Vegfaa in the absence of injury also upregulates mdm2 and suppresses Tp53 levels, and tp53 mutations augment the mitogenic effects of Nrg1. mdm2 induction is spatiotemporally associated with markers of de-differentiation in injury and growth contexts, suggesting a broad role in cardiogenesis. Our findings reveal myocardial Tp53 suppression by mitogen-induced Mdm2 as a regulatory component of innate cardiac regeneration.

Nucleated red blood cells participate in myocardial regeneration in the toad Bufo Gargarizan Gargarizan

Heart regeneration is negligible in humans and mammals but remarkable in some ectotherms. Humans and mammals lack nucleated red blood cells (NRBCs), while ectotherms have sufficient NRBCs. This study used Bufo gargarizan gargarizan, a Chinese toad subspecies, as a model animal to verify our hypothesis that NRBCs participate in myocardial regeneration. NRBC infiltration into myocardium was seen in the healthy toad hearts. Heart needle-injury was used as an enlarged model of physiological cardiomyocyte loss. It recovered quickly and scarlessly. NRBC infiltration increased during the recovery. Transwell assay was done to in vitro explore effects of myocardial injury on NRBCs. In the transwell system, NRBCs could infiltrate into cardiac pieces and could transdifferentiate toward cardiomyocytes. Heart apex cautery caused approximately 5% of the ventricle to be injured to varying degrees. In the mildly to moderately injured regions, NRBC infiltration increased and myocardial regeneration started soon after the inflammatory response; the severely damaged region underwent inflammation, scarring, and vascularity before NRBC infiltration and myocardial regeneration, and recovered scarlessly in four months. NRBCs were seen in the newly formed myocardium. Enzyme-linked immunosorbent assay and Western blotting showed that the levels of tumor necrosis factor-α, interleukin- 1β, 6, and11, cardiotrophin-1, vascular endothelial growth factor, erythropoietin, matrix metalloproteinase- 2 and 9 in the serum and/or cardiac tissues fluctuated in different patterns during the cardiac injury-regeneration. Cardiotrophin-1 could induce toad NRBC transdifferentiation toward cardiomyocytes in vitro. Taken together, the results suggest that the NRBC is a cell source for cardiomyocyte renewal/regeneration in the toad; cardiomyocyte loss triggers a series of biological processes, facilitating NRBC infiltration and transition to cardiomyocytes. This finding may guide a new direction for improving human myocardial regeneration.

Targeting thymidine phosphorylase inhibition in human colorectal cancer xenografts

Human thymidine phosphorylase (hTP) is overexpressed in several solid tumors and is commonly associated with aggressiveness and unfavorable prognosis. 6-(((1,3-Dihydroxypropan-2-yl)amino)methyl)-5-iodopyrimidine-2,4(1H,3H)-dione (CPBMF-223) is a noncompetitive hTP inhibitor, which has been described as a tumor angiogenesis inhibitor. The present study investigated the effects of CPBMF-223 in a xenograft tumor induced by human colorectal carcinoma cells (HCT-116). Additionally, CPBMF-223 capacity to reduce cell migration, its toxicological profile, and pharmacokinetic characteristics, were also evaluated. The intraperitoneal treatment with CPBMF-223 markedly prevented the relative tumor growth with an efficacy similar to that observed for 5-fluorouracil. Interestingly, number of vessels were significantly decreased in the treated groups. Moreover, CPBMF-223 significantly reduced the migration of cell line HCT-116. In the Ames assay and in an acute oral toxicity test, the molecule did not alter any evaluated parameter. Using the zebrafish toxicity model, cardiac and locomotor parameters were slightly changed. Regarding the pharmacokinetics profile, CPBMF-223 showed clearance of 9.42 L/h/kg after intravenous administration, oral bioavailability of 13.5%, and a half-life of 0.75 h. Our findings shed new light on the role of hTP in colorectal cancer induced by HCT-116 cell in mice, pointing out CPBMF-223 as, hopefully, a promising drug candidate.

The Zebrafish Cardiac Endothelial Cell-Roles in Development and Regeneration

In zebrafish, the spatiotemporal development of the vascular system is well described due to its stereotypical nature. However, the cellular and molecular mechanisms orchestrating post-embryonic vascular development, the maintenance of vascular homeostasis, or how coronary vessels integrate into the growing heart are less well studied. In the context of cardiac regeneration, the central cellular mechanism by which the heart regenerates a fully functional myocardium relies on the proliferation of pre-existing cardiomyocytes; the epicardium and the endocardium are also known to play key roles in the regenerative process. Remarkably, revascularisation of the injured tissue occurs within a few hours after cardiac damage, thus generating a vascular network acting as a scaffold for the regenerating myocardium. The activation of the endocardium leads to the secretion of cytokines, further supporting the proliferation of the cardiomyocytes. Although epicardium, endocardium, and myocardium interact with each other to orchestrate heart development and regeneration, in this review, we focus on recent advances in the understanding of the development of the endocardium and the coronary vasculature in zebrafish as well as their pivotal roles in the heart regeneration process.