Tbx20 Is an Essential Regulator of Embryonic Heart Growth in Zebrafish

A review of protocols for engineering human cardiac organoids

The use of human cardiac organoids (hCOs) as 3D in vitro models for cardiovascular research has shown great promise. Human pluripotent stem cells (hPSCs) have proven to be a potent source for engineering hCOs. However, various protocols for generating hCOs from hPSCs result in significant differences in heart development, maturity, complexity, vascularization, and spatial structure, all of which can influence their functional and physiological properties. This protocol review aims to highlight different strategies for generating hCOs using hPSCs while also critically discussing their challenges and limitations.

Hdac1-deficiency affects the cell cycle axis Cdc25-Cdk1 causing impaired G2/M phase progression and reduced cardiomyocyte proliferation in zebrafish

Zebrafish have the ability to fully regenerate their hearts after injury since cardiomyocytes subsequently dedifferentiate, re-enter cell cycle, and proliferate to replace damaged myocardial tissue. Recent research identified the reactivation of dormant developmental pathways during cardiac regeneration in adult zebrafish, suggesting pro-proliferative pathways important for developmental heart growth to be also critical for regenerative heart growth after injury. Histone deacetylase 1 (Hdac1) was recently shown to control both, embryonic as well as adult regenerative cardiomyocyte proliferation in the zebrafish model. Nevertheless, regulatory pathways controlled by Hdac1 are not defined yet. By analyzing RNA-seq-derived transcriptional profiles of the Hdac1-deficient zebrafish mutant baldrian, we here identified DNA damage response (DDR) pathways activated in baldrian mutant embryos. Surprisingly, although the DDR signaling pathway was transcriptionally activated, we found the complete loss of protein expression of the known DDR effector and cell cycle inhibitor p21. Consequently, we observed an upregulation of the p21-downstream target Cdk2, implying elevated G1/S phase transition in Hdac1-deficient zebrafish hearts. Remarkably, Cdk1, another p21-but also Cdc25-downstream target was downregulated. Here, we found the significant downregulation of Cdc25 protein expression, explaining reduced Cdk1 levels and suggesting impaired G2/M phase progression in Hdac1-deficient zebrafish embryos. To finally prove defective cell cycle progression due to Hdac1 loss, we conducted Cytometer-based cell cycle analyses in HDAC1-deficient murine HL-1 cardiomyocytes and indeed found impaired G2/M phase transition resulting in defective cardiomyocyte proliferation. In conclusion, our results suggest a critical role of Hdac1 in maintaining both, regular G1/S and G2/M phase transition in cardiomyocytes by controlling the expression of essential cell cycle regulators such as p21 and Cdc25.

Effects of constitutively active IKKβ on cardiac development

NF-κB is a major transcription factor regulating cell survival, organ development and inflammation, but its role in cardiac development has been inadequately explored. To examine this function, we generated mice in which IKKβ, an essential kinase for NF-κB activation, was constitutively activated in embryonic cardiomyocytes. For this purpose, we used smooth muscle-22α (SM22α)-Cre mice, which are frequently used for gene recombination in embryonic cardiomyocytes. Embryonic hearts of SM22αCre-CA (constitutively active) IKKβ^flox/flox mice revealed remarkably thin, spongy and hypoplastic myocardium. In exploring the mechanism, we found that the expression of bone morphogenetic protein 10 (BMP10) and T-box transcription factor 20 (Tbx20), major regulators of cardiac development, was significantly downregulated and upregulated, respectively, in the SM22αCre-CAIKKβ^flox/flox mice. We also generated NK2 homeobox 5 (Nkx2.5) Cre-CAIKKβ^flox/wt mice since Nkx2.5 is also expressed in embryonic cardiomyocytes and confirmed that the changes in these genes were also observed. These results implicated that the activation of NF-κB affects cardiac development.

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.

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.

The Role of Tbx20 in Cardiovascular Development and Function

Tbx20 is a member of the Tbx1 subfamily of T-box-containing genes and is known to play a variety of fundamental roles in cardiovascular development and homeostasis as well as cardiac remodeling in response to pathophysiological stresses. Mutations in TBX20 are widely associated with the complex spectrum of congenital heart defects (CHDs) in humans, which includes defects in chamber septation, chamber growth, and valvulogenesis. In addition, genetic variants of TBX20 have been found to be associated with dilated cardiomyopathy and heart arrhythmia. This broad spectrum of cardiac morphogenetic and functional defects is likely due to its broad expression pattern in multiple cardiogenic cell lineages and its critical regulation of transcriptional networks during cardiac development. In this review, we summarize recent findings in our general understanding of the role of Tbx20 in regulating several important aspects of cardiac development and homeostasis and heart function.

Streamlining drug discovery assays for cardiovascular disease using zebrafish

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

Gene profiling of head mesoderm in early zebrafish development: insights into the evolution of cranial mesoderm

Background

The evolution of the head was one of the key events that marked the transition from invertebrates to vertebrates. With the emergence of structures such as eyes and jaws, vertebrates evolved an active and predatory life style and radiated into diversity of large-bodied animals. These organs are moved by cranial muscles that derive embryologically from head mesoderm. Compared with other embryonic components of the head, such as placodes and cranial neural crest cells, our understanding of cranial mesoderm is limited and is restricted to few species.

Results

Here, we report the expression patterns of key genes in zebrafish head mesoderm at very early developmental stages. Apart from a basic anterior–posterior axis marked by a combination of pitx2 and tbx1 expression, we find that most gene expression patterns are poorly conserved between zebrafish and chick, suggesting fewer developmental constraints imposed than in trunk mesoderm. Interestingly, the gene expression patterns clearly show the early establishment of medial–lateral compartmentalisation in zebrafish head mesoderm, comprising a wide medial zone flanked by two narrower strips.

Conclusions

In zebrafish head mesoderm, there is no clear molecular regionalisation along the anteroposterior axis as previously reported in chick embryos. In contrast, the medial–lateral regionalisation is formed at early developmental stages. These patterns correspond to the distinction between paraxial mesoderm and lateral plate mesoderm in the trunk, suggesting a common groundplan for patterning head and trunk mesoderm. By comparison of these expression patterns to that of amphioxus homologues, we argue for an evolutionary link between zebrafish head mesoderm and amphioxus anteriormost somites.

Electronic supplementary material

The online version of this article (10.1186/s13227-019-0128-3) contains supplementary material, which is available to authorized users.

Conditional mutagenesis by oligonucleotide-mediated integration of loxP sites in zebrafish

Many eukaryotic genes play essential roles in multiple biological processes in several different tissues. Conditional mutants are needed to analyze genes with such pleiotropic functions. In vertebrates, conditional gene inactivation has only been feasible in the mouse, leaving other model systems to rely on surrogate experimental approaches such as overexpression of dominant negative proteins and antisense-based tools. Here, we have developed a simple and straightforward method to integrate loxP sequences at specific sites in the zebrafish genome using the CRISPR/Cas9 technology and oligonucleotide templates for homology directed repair. We engineered conditional (floxed) mutants of tbx20 and fleer, and demonstrate excision of exons flanked by loxP sites using tamoxifen-inducible CreERT2 recombinase. To demonstrate broad applicability of our method, we also integrated loxP sites into two additional genes, aldh1a2 and tcf21. The ease of this approach will further expand the use of zebrafish to study various aspects of vertebrate biology, especially post-embryonic processes such as regeneration.

Some genes are expressed and function in a single tissue, and the effect of their loss on that tissue can be readily determined. Frequently, however, genes that are necessary for the development or maintenance of one tissue are also important for other tissues or cell types. Genes of the latter type are difficult to analyze because of the complications resulting from an organism having multiple defects in different tissues. The solution pioneered by mouse geneticists is to inactivate the gene of interest in only one tissue at a time. This elegant approach requires the ability to make specific edits to the genome, a technology that was not readily available to zebrafish researchers until recently. Using the CRISPR/Cas9 genome editing tool, we have developed a simple, reliable, and efficient method to insert DNA sequences into the zebrafish genome that enable conditional gene inactivation. Our methodology will be useful not only for the study of genes that play important roles in multiple tissues, but also for the genetic analysis of biological processes which occur in adult animals.

Loss of the novel Vcp (valosin containing protein) interactor Washc4 interferes with autophagy-mediated proteostasis in striated muscle and leads to myopathy in vivo

VCP/p97 (valosin containing protein) is a key regulator of cellular proteostasis. It orchestrates protein turnover and quality control in vivo, processes fundamental for proper cell function. In humans, mutations in VCP lead to severe myo- and neuro-degenerative disorders such as inclusion body myopathy with Paget disease of the bone and frontotemporal dementia (IBMPFD), amyotrophic lateral sclerosis (ALS) or and hereditary spastic paraplegia (HSP). We analyzed here the in vivo role of Vcp and its novel interactor Washc4/Swip (WASH complex subunit 4) in the vertebrate model zebrafish (Danio rerio). We found that targeted inactivation of either Vcp or Washc4, led to progressive impairment of cardiac and skeletal muscle function, structure and cytoarchitecture without interfering with the differentiation of both organ systems. Notably, loss of Vcp resulted in compromised protein degradation via the proteasome and the macroautophagy/autophagy machinery, whereas Washc4 deficiency did not affect the function of the ubiquitin-proteasome system (UPS) but caused ER stress and interfered with autophagy function in vivo. In summary, our findings provide novel insights into the in vivo functions of Vcp and its novel interactor Washc4 and their particular and distinct roles during proteostasis in striated muscle cells.

Fish, the better model in human heart research? Zebrafish Heart aggregates as a 3D spontaneously cardiomyogenic in vitro model system

The zebrafish (ZF) has become an essential model for biomedical, pharmacological and eco-toxicological heart research. Despite the anatomical differences between fish and human hearts, similarities in cellular structure and conservation of genes as well as pathways across vertebrates have led to an increase in the popularity of ZF as a model for human cardiac research. ZF research benefits from an entirely sequenced genome, which allows us to establish and study cardiovascular mutants to better understand cardiovascular diseases. In this review, we will discuss the importance of in vitro model systems for cardiac research and summarise results of in vitro 3D heart-like cell aggregates, consisting of myocardial tissue formed spontaneously from enzymatically digested whole embryonic ZF larvae (Zebrafish Heart Aggregate - ZFHA). We will give an overview of the similarities and differences of ZF versus human hearts and highlight why ZF complement established mammalian models (i.e. murine and large animal models) for cardiac research. At this stage, the ZFHA model system is being refined into a high-throughput (more ZFHA generated than larvae prepared) and stable in vitro test system to accomplish the same longevity of previously successful salmonid models. ZFHA have potential for the use of high-throughput-screenings of different factors like small molecules, nucleic acids, proteins and lipids which is difficult to achieve in the zebrafish in vivo screening models with lethal mutations as well as to explore ion channel disorders and to find appropriate drugs for safety screening.